Short answer
Guru Nanak says the word of the Lord came to him, and Sikhs honour the Guru Granth Sahib, their living Guru, as that Word. Islam does not judge the Gurus' hearts or call them liars, but it does not count the Granth as revelation. Its reason rests on one question: is the Qur'an from God (﷾)? The Qur'an invites everyone to test it, by trying to produce a chapter like it (2:23) and by looking for discrepancy in it (4:82). If it passes, its word that Muhammad (ﷺ) is the last prophet is God's (﷾) word, and no revelation comes after him. The full explanation sets out each step.
Full explanation
What the Gurus say about the Word
Guru Nanak (Sikhs say Guru Nanak Dev Ji) begins a hymn at Ang 722 with a line about the word that comes to him. The hymn is in the Guru Granth Sahib. An Ang is a page of the standard 1,430-page edition, and Sikhs honour the Guru Granth Sahib as their living Guru. The hymn is his lament over Babur's invasion, and only its first line is quoted here. Guru Arjan, the fifth Guru, says at Ang 628 that the bani (the hymn, the Word) came from the Primal Lord. At Ang 966 the bards Satta and Balvand say that Guru Nanak and Guru Angad shared one Light and one way. Sikhs read this as the teaching that one Light, one Word, passes through the Gurus. This is the claim the page weighs.
“As the word of the Lord cometh to me, so I make known, O Lalo”
In a Sorath hymn Guru Arjan, the fifth Guru, says that the bani (the hymn, the Word) came from the Primal Lord, and that it removes all anxiety. Sikhs often cite this line for the teaching that Gurbani comes from God.
In their Var in the Ramkali raga, the bards Satta and Balvand say of Guru Nanak and Guru Angad, who was first called Lehna, that they shared the same Light and the same way, and that the Master only changed his body. Sikhs take this as the teaching that one Light, one Word, passes through all the Gurus.
Islam tests claims; it does not judge hearts
The Prophet (ﷺ) said that Allah (﷾) looks at hearts and deeds. Muslims therefore do not say what was in the heart of any Guru. They make no charge against the Gurus' sincerity. Islam asks what it can weigh: what a book claims, and whether the claim holds up.
Abu Huraira reports that the Prophet (ﷺ) said: Allah does not look at your faces or your wealth, but He looks at your hearts and your deeds.
How the two books present themselves
In the Qur'an God speaks in the first person. The verse that sets the challenge speaks of what "We have revealed" to "Our servant". Al-Fatiha is a prayer a servant says to God (﷾), and the Prophet (ﷺ) reported that Allah (﷾) answers it verse by verse. Muslims hold that even this prayer is words He gave. The hymns of the Guru Granth Sahib are mostly addressed to God (﷾) by named writers. They are the Gurus, the Bhagats (saints whose hymns were included) and the bards. Many are signed "Nanak", the name each Guru used. See Ang 723 (Guru Nanak) and Ang 628 (Guru Arjan). Sikhs hold that the writer is the vessel of the Word. This difference does not decide the question alone. It shows that the books make different claims about who is speaking, so each claim must be weighed.
“And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true.”
“In the name of Allah, Most Gracious, Most Merciful. Praise be to Allah, the Cherisher and Sustainer of the worlds; Most Gracious, Most Merciful; Master of the Day of Judgment. Thee do we worship, and Thine aid we seek. Show us the straight way, The way of those on whom Thou hast bestowed Thy Grace, those whose (portion) is not wrath, and who go not astray.”
Abu Hurayra reports that the Prophet (ﷺ) said a prayer without Umm al-Qur'an (al-Fatiha) is incomplete, and that he heard him say that Allah the Exalted said: I have divided the prayer between Myself and My servant in two halves, and My servant shall have what he asks. Allah then answers each verse of al-Fatiha as the servant says it: to the praise He says that His servant has praised Him, to the verse of mercy that he has extolled Him, and to the verse of the Day of Judgement that he has glorified Him; of "Thee do we worship, and Thine aid we seek" He says that it is between Him and His servant, and of the closing prayer for guidance that it is for His servant, who shall have what he asks.
At the end of his Tilang hymn on Babur's invasion, Guru Nanak says the invaders will come in the year 78 and depart in 97, and that he speaks the word of truth at its right time. Macauliffe's note (vol. 1, p. 110) reads the years as Sambat 1578 and 1597 (A.D. 1521 and 1540), the invaders as the Mughals, and the departing monarch as Humayun.
In a Sorath hymn Guru Arjan, the fifth Guru, says that the bani (the hymn, the Word) came from the Primal Lord, and that it removes all anxiety. Sikhs often cite this line for the teaching that Gurbani comes from God.
Why the general tests do not settle it here
Some of the tests on the page about testing a scripture may be passed by the Guru Granth Sahib. It says it is from God (﷾), and it teaches one Creator. This page does not say otherwise. Passing such tests shows that a book may be from God (﷾). It does not show that a later book may set aside what an earlier one commands. In Ang 1136 Guru Arjan says he keeps no fast or Ramadan and goes on no pilgrimage to Mecca. He says he does not pray as the Muslims do. Macauliffe heads the hymn "written on a theme of Kabir", and it closes by naming Kabir. The Qur'an says God (﷾) commanded prayer, fasting and the Hajj. It allows that earlier peoples had different laws (5:48). But it says its messenger was sent to all mankind (7:158; 34:28). So if the Qur'an is from God (﷾), a later word cannot set its commands aside for anyone. The question narrows to what the Qur'an stakes itself on. No one can produce its like, and a book from anyone but God (﷾) would hold much discrepancy. The page on what the Guru Granth Sahib says about Muslims gives the whole of Ang 1136.
“I practise not fasting, nor observe the Ramzan: I serve Him who will preserve me at the last hour. The one Lord of the earth is my God, Who judgeth both Hindus and Musalmans. I go not on a pilgrimage to Makka, nor worship at Hindu places of pilgrimages. I serve the one God and none other. I neither worship as the Hindus, nor pray as the Musalmans. I take the Formless God into my heart, and there make obeisance unto Him. I am neither a Hindu nor a Musalman. The soul and body belong to God whether He be called Allah or Ram. Kabir hath delivered this lecture. When I meet a true guru or pir, I recognize my own Master.”
“There is but one God whose name is true, the Creator, devoid of fear and enmity, immortal, unborn, self-existent”
“And be steadfast in prayer and regular in charity: And whatever good ye send forth for your souls before you, ye shall find it with Allah: for Allah sees Well all that ye do.”
“O ye who believe! Fasting is prescribed to you as it was prescribed to those before you, that ye may (learn) self-restraint,-”
“In it are Signs Manifest; (for example), the Station of Abraham; whoever enters it attains security; Pilgrimage thereto is a duty men owe to Allah,- those who can afford the journey; but if any deny faith, Allah stands not in need of any of His creatures.”
“To thee We sent the Scripture in truth, confirming the scripture that came before it, and guarding it in safety: so judge between them by what Allah hath revealed, and follow not their vain desires, diverging from the Truth that hath come to thee. To each among you have we prescribed a law and an open way. If Allah had so willed, He would have made you a single people, but (His plan is) to test you in what He hath given you: so strive as in a race in all virtues. The goal of you all is to Allah; it is He that will show you the truth of the matters in which ye dispute;”
“Say: "O men! I am sent unto you all, as the Messenger of Allah, to Whom belongeth the dominion of the heavens and the earth: there is no god but He: it is He That giveth both life and death. So believe in Allah and His Messenger, the Unlettered Prophet, who believeth in Allah and His words: follow him that (so) ye may be guided."”
“We have not sent thee but as a universal (Messenger) to men, giving them glad tidings, and warning them (against sin), but most men understand not.”
“And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true.”
“Say: "If the whole of mankind and Jinns were to gather together to produce the like of this Qur'an, they could not produce the like thereof, even if they backed up each other with help and support.”
“Do they not consider the Qur'an (with care)? Had it been from other Than Allah, they would surely have found therein Much discrepancy.”
The tests the Qur'an sets for itself
The Qur'an asks to be tested, and it names the tests. It tells anyone in doubt about what God (﷾) revealed to His servant to produce a sura (chapter) like it (2:23). It says that all mankind and the jinn (unseen beings) together could not produce its like (17:88). And it says that a book from anyone but God (﷾) would hold much discrepancy (4:82). These are the Qur'an's claims about itself. Islam holds that it passes them, and the pages on the Qur'an's challenge and on contradictions give the evidence. This page uses only the tests the Qur'an sets for itself, which Islam holds it passes. It does not say the Guru Granth Sahib fails them. It asks the reader to test the Qur'an first, because what the Qur'an says about the last Prophet depends on whether it is God's (﷾) word.
“And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true.”
“Say: "If the whole of mankind and Jinns were to gather together to produce the like of this Qur'an, they could not produce the like thereof, even if they backed up each other with help and support.”
“Do they not consider the Qur'an (with care)? Had it been from other Than Allah, they would surely have found therein Much discrepancy.”
Then what the Qur'an says about the last Prophet
Only after that test does the Qur'an's verse on the last Prophet carry weight. To a Sikh reader it is the very point in question. The Qur'an calls Muhammad (ﷺ) the Seal of the Prophets (33:40) and says God (﷾) has perfected the religion (5:3). The Prophet (ﷺ) said there would be no prophet after him. A Sikh may reply that the Gurus are not prophets in this sense. Whatever title is used, Ang 722 and Ang 628 claim a word that comes from God (﷾), and that is a claim of revelation. If the Qur'an is from God (﷾), its word on the completed religion applies to every such claim, from any people. The Qur'an also says every people had a warner (35:24). Those warners came before the Seal of the Prophets. Guru Nanak was born in A.D. 1469, long after, so Islam does not count him among them.
“Muhammad is not the father of any of your men, but (he is) the Messenger of Allah, and the Seal of the Prophets: and Allah has full knowledge of all things.”
“This day have I perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion.”
Abu Hurayra reports that the Prophet (ﷺ) said the Israelites were led by prophets, and whenever one died another took his place. But there will be no prophet after me, he said; there will be caliphs, and many of them.
“Verily We have sent thee in truth, as a bearer of glad tidings, and as a warner: and there never was a people, without a warner having lived among them (in the past).”
Macauliffe records that Guru Nanak was born on the third day of the light half of Baisakh in the year 1526 of the Vikramaditya era, which he gives as A.D. 1469.
Islam's invitation
Islam asks a Sikh reader for what it asks of anyone. Read the Qur'an in a translation you trust, and ask whether it is from God (﷾). Try its challenge and look for the discrepancy it says you will not find. If it is God's (﷾) word, then following what it says about the last Prophet (ﷺ) is the way to follow Him.
“And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true.”
“Do they not consider the Qur'an (with care)? Had it been from other Than Allah, they would surely have found therein Much discrepancy.”
Line by line
Guru Granth Sahib, Ang 723 — Raga Tilang, Guru Nanak
At the end of his Tilang hymn on Babur's invasion, Guru Nanak says the invaders will come in the year 78 and depart in 97, and that he speaks the word of truth at its right time. Macauliffe's note (vol. 1, p. 110) reads the years as Sambat 1578 and 1597 (A.D. 1521 and 1540), the invaders as the Mughals, and the departing monarch as Humayun.
Cited for
The Guru Granth Sahib passes your tests: it says it is from God, and Ang 723 foretold when the Mughals would come and go.
The response
Islam does not dispute that Sikhs read the line as foresight, and a foretold event is worth weighing in any book. But it cannot decide between two books when one sets aside commands the other sent to all mankind (7:158). The tests used here are the Qur'an's own: its challenge (2:23) and its call to look for discrepancy (4:82). This page does not say the Guru Granth Sahib fails them.
Where it lands — Signs such as foresight are worth weighing. But if the Qur'an is God's (﷾) word, no later book can set aside the commands it sent to all mankind. The deciding question is whether it is His word, and it invites every reader to test it.
Qur'an 2:23 — Surat al-Baqara (The Cow), verse 23
“And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true.”
Cited for
Gurbani cannot be matched either, so the Qur'an's challenge proves nothing special.
The response
Islam does not dispute how highly Sikhs hold Gurbani. The Qur'an makes its challenge its own public proof, put to its opponents in its own words (2:23; 17:88). A claim a book makes for itself can be tried by anyone. Islam holds that the Qur'an has stood it.
Where it lands — The Qur'an is tested by the challenge it set itself, and that is why it is tested first.
Guru Granth Sahib, Ang 966 — Raga Ramkali, the bards Satta and Balvand
In their Var in the Ramkali raga, the bards Satta and Balvand say of Guru Nanak and Guru Angad, who was first called Lehna, that they shared the same Light and the same way, and that the Master only changed his body. Sikhs take this as the teaching that one Light, one Word, passes through all the Gurus.
Cited for
Gurbani is not a prophet's message. It is the one Divine Word, the same Light in the Gurus' bodies, so the Seal of the Prophets does not apply.
The response
Islam does not dispute how Sikhs describe the unity of the Gurus. It sets one difference beside it. In the Qur'an God speaks in the first person to one messenger, and the book challenges readers to match a chapter (2:23; 17:88). The Qur'an's description of itself comes with a test.
Where it lands — The two books describe their origin differently, and the Qur'an's description invites a test. So the Qur'an is tested first.
Qur'an 33:40 — Surat al-Ahzab (The Clans), verse 40
“Muhammad is not the father of any of your men, but (he is) the Messenger of Allah, and the Seal of the Prophets: and Allah has full knowledge of all things.”
Cited for
The Gurus do not claim to be prophets, so the Seal of the Prophets does not apply to them.
The response
Whatever title is used, Ang 722 and Ang 628 claim a word that comes from God (﷾), and that is a claim of revelation. The Qur'an says God (﷾) perfected the religion (5:3) and calls Muhammad (ﷺ) the Seal of the Prophets (33:40). The claim is weighed against that, once the Qur'an has been tested.
Where it lands — The claim is one of revelation either way. It is weighed against the Qur'an's own word on the last Prophet, after the Qur'an has been tested.
Qur'an 5:3 — Surat al-Ma'ida (The Table), verse 3
“This day have I perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion.”
Cited for
God is not limited. If He spoke to earlier peoples, He can speak again, so the Qur'an cannot close the door.
The response
Islam does not say God (﷾) is limited. The question is what He has told us. The Qur'an says He perfected the religion (5:3) and calls Muhammad (ﷺ) the Seal of the Prophets (33:40). That testimony is God's (﷾) word only if the Qur'an is, so the Qur'an is tested first.
Where it lands — What God (﷾) can do is not in doubt. What He has said is the question, and the Qur'an's answer stands or falls with the Qur'an's test.
Qur'an 1:1-7 — Surat al-Fatiha (The Opening), verses 1-7
“In the name of Allah, Most Gracious, Most Merciful. Praise be to Allah, the Cherisher and Sustainer of the worlds; Most Gracious, Most Merciful; Master of the Day of Judgment. Thee do we worship, and Thine aid we seek. Show us the straight way, The way of those on whom Thou hast bestowed Thy Grace, those whose (portion) is not wrath, and who go not astray.”
Cited for
Al-Fatiha is a prayer said to God. So the Qur'an has human voices in it, as the Guru Granth Sahib has.
The response
The Qur'an does contain a prayer said to God (﷾), and Muslims hold that He gave its words. The Prophet (ﷺ) reported that Allah (﷾) divides the prayer between Himself and His servant and answers each verse as the servant says it (Muslim 395a). Even in the Qur'an's prayer, the words are held to come from God (﷾).
Where it lands — A prayer inside the Qur'an is held to be words God gave, so it does not make the book a human one.
Perspectives
Sikh: the bani is the Word of God, spoken through the Gurus
Sikhs hold that Guru Nanak and the Gurus after him uttered the Word as it came to them from God. The Guru Granth Sahib is that Word, one Light shining in ten Gurus. Sikhs honour it as the living Guru.
Guru Nanak begins a hymn at Ang 722 with a statement. He says that he makes known the Lord's word as it comes to him.
Rebuttal
The hymn says this, and Islam does not dispute that Guru Nanak understood his words so. A claim of this kind does not itself decide whether revelation continued after the Qur'an. The Qur'an asks to be tested first (2:23; 4:82).
Guru Arjan says at Ang 628 that the bani came from the Primal Lord, so Gurbani is God's word.
Rebuttal
Many books say they are from God (﷾), so the saying alone settles nothing. The Qur'an does not rest on the saying alone. It adds a challenge to produce a chapter like it and a call to look for discrepancy.
The bards Satta and Balvand say the Gurus shared one Light and one way. Sikhs hold that Gurbani is one Word spoken through all of them, not a prophet's message.
Rebuttal
Islam does not dispute how Sikhs describe the unity of the Gurus. It asks a different question: what does the Qur'an claim, and is the claim true? The Qur'an offers its own test (2:23; 17:88).
Sikh readers take Ang 723 as foresight. Guru Nanak says they would come in the year 78 and leave in 97. Macauliffe reads this as the Mughals, and as Sambat (the Vikram calendar) 1578 and 1597, that is A.D. 1521 and 1540.
Rebuttal
Take the reading as given. A fulfilled prophecy is worth weighing in any book. But the Qur'an's messenger was sent to all mankind (7:158), so if it is God's (﷾) word, no later book can set its commands aside. That is why the Qur'an is tested first (2:23; 4:82).
By the general tests of a scripture, the Guru Granth Sahib passes. It says it is from God, and it teaches one Creator.
Rebuttal
Islam does not say the Guru Granth Sahib fails them. But the Qur'an's messenger was sent to all mankind (7:158), and Ang 1136 sets its commands aside. So the deciding tests are the Qur'an's own: its challenge (2:23) and its call to look for discrepancy (4:82).
The Islamic perspective
Islam does not judge the Gurus' hearts or call them liars. It tests claims, beginning with the Qur'an's own, and holds that the Qur'an, which passes the tests it sets for itself, completed God's (﷾) scripture.
Muslims hold that God (﷾) looks at hearts, so Islam does not judge whether a Guru was sincere. It weighs what a book claims.
The Qur'an presents itself as what God (﷾) revealed to His servant. It tells anyone in doubt to produce a sura like it.
The Qur'an says that a book from anyone but God (﷾) would hold much discrepancy. It invites the reader to look.
Al-Fatiha is a prayer said to God (﷾), yet it stands in the Qur'an as His word. The Prophet (ﷺ) reported that Allah (﷾) answers each verse as His servant says it.
If the Qur'an is from God (﷾), it closes the line of revelation. It says that Muhammad (ﷺ) is the Seal of the Prophets and that the religion is perfected.
The Qur'an says every people had a warner. All came before the Seal of the Prophets, so Islam does not count Guru Nanak among them.
In Ang 1136 Guru Arjan keeps no fast or Ramadan and goes on no pilgrimage to Mecca. The Qur'an says God (﷾) commanded prayer, fasting and the Hajj, and that its messenger was sent to all mankind. So if the Qur'an is God's (﷾) word, a later hymn cannot set those commands aside.