Short answer
The NIV, a modern English Bible, leaves out hundreds of phrases and sixteen verses found in the King James Version (KJV). Verses such as Acts 8:37 appear only in its footnotes. It prints two whole passages with a note that the earliest manuscripts do not have them. They are the last twelve verses of Mark and the story of the woman caught in adultery. It also drops words like "through his blood" and the closing line of the Lord's Prayer, "For thine is the kingdom". Thirteen of these omissions fall in Revelation, the book that curses anyone who adds to its words or takes from them. Christians argue over which Bible is right. Either way, copyists changed the text: the KJV prints words the oldest copies lack, or the modern Bibles leave God's (﷾) words out. Islam's answer: God (﷾) sent the Qur'an down and promised to guard it.
Full explanation
The two passages in question
Mark 16:9-20 is the last twelve verses of Mark. John 7:53-8:11 is the story of the woman caught in adultery. The King James Version prints both as scripture. The NIV prints both with notes that the earliest manuscripts, the hand-copied ancient books, do not have them. Scholars report that neither is found in the two oldest near-complete Greek Bibles. They also report that the story of the adulteress turns up in different places in the manuscripts that have it.
“Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.”
“So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.”
The NIV prints Mark 16:9-20 set apart, with a note that the earliest manuscripts and some other ancient witnesses do not have these verses (1984: "the most reliable early manuscripts and other ancient witnesses"). The NIV Study Bible adds that serious doubts exist whether the verses belong to Mark, because they are missing from important early manuscripts and differ in vocabulary, style and theology, and that the Gospel probably ended at 16:8 or its first ending was lost.
On Mark 16:9-20, reports that the last twelve verses are absent from the two oldest Greek Bibles that survive in large part, Codex Sinaiticus and Codex Vaticanus, and from other early witnesses; that Eusebius and Jerome state the verses were missing from almost all the Greek copies known to them; that some manuscripts carry a different, shorter ending instead; and concludes that the Gospel as we have it from the earliest evidence ends at 16:8.
On the story of the woman taken in adultery (John 7:53-8:11), reports that it is absent from the early papyri (P66 and P75), from Codex Sinaiticus and Codex Vaticanus and other early witnesses, and that the manuscripts which do carry it place it in different spots: after John 7:52, after John 7:36, at the end of John, or after Luke 21:38. Concludes that it was no part of the original Gospel of John.
Sixteen verses moved to footnotes
Sixteen verses of the King James Version appear in the NIV only in footnotes. One is the Ethiopian official's confession in Acts 8:37 that Jesus Christ is the Son of God. The oldest manuscripts do not have it.
The New International Version leaves sixteen verses that stand in the King James Version out of its main text and gives them only in footnotes: Matthew 17:21, 18:11 and 23:14; Mark 7:16, 9:44, 9:46, 11:26 and 15:28; Luke 17:36 and 23:17; John 5:4; Acts 8:37, 15:34, 24:7 and 28:29; and Romans 16:24. It prints Mark 16:9-20 and John 7:53-8:11 with notes that the earliest manuscripts do not have them. It ends the Lord's Prayer at Matthew 6:13 without "for yours is the kingdom and the power and the glory forever. Amen", noting that only some late manuscripts add it. At Colossians 1:14 it reads "redemption, the forgiveness of sins", without "through his blood". In Revelation it lacks thirteen words or phrases found in the King James Version, at 1:8, 1:11 (twice), 5:14, 11:1, 11:17, 14:12, 15:2, 16:5, 16:7, 16:14, 19:1 and 21:24.
“And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.”
On Acts 8:37, judges the verse a later addition. It is missing from the oldest witnesses, among them papyri 45 and 74 and the codices Sinaiticus, Alexandrinus, Vaticanus and Ephraemi, and there is no reason a scribe would have left it out had it been original. He suggests a confession of faith used at baptism was written in the margin and later copied into the text.
Words missing within verses
Hundreds of smaller phrases differ. The closing praise of the Lord's Prayer is missing from the NIV's Matthew. "Through his blood" stands in the King James Colossians 1:14 and is absent from the public-domain World English Bible. The King James words about three witnesses in heaven in 1 John 5:7 are not in the modern text at all. In 1 Timothy 3:16 the King James reads "God was manifest in the flesh". The Revised Version of 1881 and the modern Bibles after it read "He who" or "He".
“And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen.”
The New International Version leaves sixteen verses that stand in the King James Version out of its main text and gives them only in footnotes: Matthew 17:21, 18:11 and 23:14; Mark 7:16, 9:44, 9:46, 11:26 and 15:28; Luke 17:36 and 23:17; John 5:4; Acts 8:37, 15:34, 24:7 and 28:29; and Romans 16:24. It prints Mark 16:9-20 and John 7:53-8:11 with notes that the earliest manuscripts do not have them. It ends the Lord's Prayer at Matthew 6:13 without "for yours is the kingdom and the power and the glory forever. Amen", noting that only some late manuscripts add it. At Colossians 1:14 it reads "redemption, the forgiveness of sins", without "through his blood". In Revelation it lacks thirteen words or phrases found in the King James Version, at 1:8, 1:11 (twice), 5:14, 11:1, 11:17, 14:12, 15:2, 16:5, 16:7, 16:14, 19:1 and 21:24.
“In whom we have redemption through his blood, even the forgiveness of sins:”
“in whom we have our redemption, the forgiveness of our sins.”
“For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.”
“For there are three who testify:”
On 1 John 5:7-8, reports that the words about the Father, the Word and the Holy Spirit are absent from every known Greek manuscript except eight late ones, four of which have them written in the margin as a later addition; that no Greek church father quotes them; and that every ancient translation except the Latin lacks them.
“And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.”
The English Revised Version (1881) and the American Standard Version (1901), both public domain, follow the older manuscripts. Both begin the line "He who was manifested in the flesh" and do not have the word "God".
The King James Version and modern Bibles such as the NIV do not say the same thing. Whole verses are in one and missing from the other.
If the King James is right?
Then the modern Bibles leave God's (﷾) words out.
If the modern Bibles are right?
Then the King James prints words the oldest copies lack, such as the three witnesses in heaven in 1 John 5:7.
Either way, copyists changed the text of the Bible.
Changes inside the book that forbids them
Revelation ends with a curse on anyone who adds to its words or takes from them. In the King James Version Jesus says "I am Alpha and Omega, the first and the last" in 1:11. The Lord says "the beginning and the ending" in 1:8. The World English Bible has neither phrase at those verses. Proverbs forbids adding to God's (﷾) words. Deuteronomy forbids both adding to and taking from the commandments of the LORD that Moses delivered.
“For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.”
“I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.”
““I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.””
“Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.”
“saying, “What you see, write in a book and send to the seven assemblies: to Ephesus, Smyrna, Pergamum, Thyatira, Sardis, Philadelphia, and to Laodicea.””
“Add thou not unto his words, lest he reprove thee, and thou be found a liar.”
“Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you.”
What the Qur'an says about this
The Qur'an speaks of people who wrote the Book with their own hands and said "This is from Allah". It also speaks of others who read out words as scripture that were not from God (﷾). It says of itself that God (﷾) sent it down and will guard it.
“Then woe to those who write the Book with their own hands, and then say:"This is from Allah," to traffic with it for miserable price!- Woe to them for what their hands do write, and for the gain they make thereby.”
“There is among them a section who distort the Book with their tongues: (As they read) you would think it is a part of the Book, but it is no part of the Book; and they say, "That is from Allah," but it is not from Allah: It is they who tell a lie against Allah, and (well) they know it!”
“We have, without doubt, sent down the Message; and We will assuredly guard it (from corruption).”
Point by point
A Textual Commentary on the Greek New Testament (2nd ed., 1994), on Mark 16:9-20
On Mark 16:9-20, reports that the last twelve verses are absent from the two oldest Greek Bibles that survive in large part, Codex Sinaiticus and Codex Vaticanus, and from other early witnesses; that Eusebius and Jerome state the verses were missing from almost all the Greek copies known to them; that some manuscripts carry a different, shorter ending instead; and concludes that the Gospel as we have it from the earliest evidence ends at 16:8.
Cited for
Nothing was removed. The oldest manuscripts never had these words. Modern Bibles restore the original.
The response
Then for centuries the church read, printed and preached as the word of God (﷾) words the oldest copies lack. The King James Version still does. That is the very thing Revelation and Proverbs forbid.
Where it lands — On this reply, the church read as God's (﷾), for centuries, words the oldest copies lack.
1 John 5:7 — First Epistle of John, chapter 5, verse 7
“For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.”
Cited for
No doctrine depends on the disputed verses. Every teaching is still found elsewhere.
The response
The question is not whether a doctrine survives but whether God's (﷾) words were kept. Jesus said man lives by every word of God (﷾). The words in 1 John 5:7 say that the Father, the Word and the Holy Ghost are one. In the King James Bible they were the clearest Trinity verse. They appear in only eight late Greek manuscripts, four of them written in the margin. No Greek church father quotes them.
Where it lands — Even if no doctrine is lost, words the oldest copies lack were printed as God's (﷾) for centuries.
Perspectives
Modern-text Christian
The modern Bibles follow the oldest and best manuscripts. The missing words are not in the oldest and best manuscripts. Leaving them out restores what the first writers wrote, and no teaching of the faith is lost.
The oldest Greek manuscripts lack the ending of Mark and the story of the adulteress. So, they say, modern Bibles are right to mark them.
Rebuttal
The oldest manuscripts do lack these passages, and that is granted. But the King James Version printed them as scripture for centuries, and Christians still disagree over which Bible is right. So for centuries the church read as God's (﷾) word what the oldest copies lack.
King James Only Christian
God promised to preserve every word. The King James Version, from the Received Text, keeps them, and the modern Bibles have cut God's word out.
Jesus said man lives by every word from the mouth of God. So a Bible that drops words has lost God's words.
Rebuttal
Jesus did say man lives by every word of God (﷾), and that is granted. But the King James Version has words in 1 John 5:7 that stand in only eight late Greek manuscripts. No Greek church father quotes them. A promise of every word cannot point to words the Greek church never knew.
The Islamic perspective
Both Christian sides admit that copyists changed the text. Either words the oldest copies lack were printed as God's (﷾) word for centuries, or God's (﷾) words were left out. The Qur'an was sent down and kept, as the Prophet (ﷺ) delivered it.
The NIV moves sixteen verses that the King James Version prints as scripture into footnotes. It marks two whole passages as missing from the earliest manuscripts.
Public-domain Bibles on this site differ word for word. Examples are "through his blood" in Colossians 1:14 and "Alpha and Omega" in Revelation 1:11.
Revelation curses anyone who adds to its words or takes from them. Yet its own text differs between the King James and modern Bibles.
Whichever text is right, copyists changed the words, which Proverbs and Deuteronomy forbid.
The Qur'an says some wrote scripture with their own hands and called it God's (﷾). It says God (﷾) will guard the Qur'an.