Short answer
God (﷾) did not need to wait, because He never needed a death in order to forgive. The Qur'an says that Adam repented after eating from the tree, and God (﷾) accepted his repentance. No sacrifice was needed, and no one else paid for his sin. On the Christian account, God (﷾) knew from Adam's day that people would sin, yet the remedy came thousands of years later. The Bible itself shows God (﷾) forgiving people who turned back to Him, long before the cross Christians speak of. And God (﷾), who is almighty, can forgive by His will alone. In Islam, a believer's hardship also has a purpose. This life is a test, and God (﷾) wipes away sins through the hardships a believer bears.
Full explanation
Adam repented and was forgiven
The Qur'an tells the story of Adam without any debt left waiting. Adam and his wife admitted their wrong and asked for forgiveness. God (﷾) taught Adam words of repentance, turned to him, chose him and guided him. The sin ended where it began, and no one else was asked to pay for it.
“They said: "Our Lord! We have wronged our own souls: If thou forgive us not and bestow not upon us Thy Mercy, we shall certainly be lost."”
“Then learnt Adam from his Lord words of inspiration, and his Lord Turned towards him; for He is Oft-Returning, Most Merciful.”
“But his Lord chose him (for His Grace): He turned to him, and gave him Guidance.”
The way back was open to every people
The Qur'an says every people on earth had a warner. Every messenger brought the same call: serve Allah (﷾) and turn away from evil. So no people between Adam and Jesus was left without a way back to God (﷾).
“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).”
“For We assuredly sent amongst every People a messenger, (with the Command), "Serve Allah, and eschew Evil": of the People were some whom Allah guided, and some on whom error became inevitably (established). So travel through the earth, and see what was the end of those who denied (the Truth).”
What Christians say
Christians answer that God (﷾) sent His Son at the moment He had chosen, "the fulness of the time". Paul, a Christian missionary of the first century, writes of "the remission of sins that are past, through the forbearance of God". Remission means forgiveness, and forbearance means patience. Many read this as forgiveness in advance for those who lived before the cross. On suffering, Paul says believers still groan while they wait for "the redemption of our body".
“But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,”
“Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;”
“And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.”
Forgiveness in the Bible without a cross
If no one could be forgiven without the cross Christians speak of, the generations before Jesus would have had no hope. The Bible shows the opposite. Ezekiel, a Hebrew prophet of the sixth century BCE, says the wicked man who turns from his sins will live. Nineveh, the city Jonah warned, turned from its evil and was spared. Jesus says the tax collector who asked for mercy went home justified. Each passage gives its own reason, the sinner's turning back, and none mentions a death still to come.
“But if the wicked will turn from all his sins that he hath committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die. All his transgressions that he hath committed, they shall not be mentioned unto him: in his righteousness that he hath done he shall live.”
“And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not.”
“And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner. I tell you, this man went down to his house justified rather than the other: for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted.”
The power to forgive belongs to God (﷾)
The Bible calls God Almighty and says nothing is impossible for Him, so forgiveness rests on His will. The New Testament Gospels show Jesus, while he was still preaching, telling a paralysed man his sins were forgiven. Matthew says the crowd glorified God (﷾), who had given such power to men.
“And when Abram was ninety years old and nine, the Lord appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.”
“For with God nothing shall be impossible.”
“When Jesus saw their faith, he said unto the sick of the palsy, Son, thy sins be forgiven thee.”
“But when the multitudes saw it, they marvelled, and glorified God, which had given such power unto men.”
Why the forgiven still suffer
Many Christians, especially Protestants, hold that the cross paid for every sin once for all. Protestants are Christians whose churches broke from the Catholic Church in the sixteenth century. On this view, a believer's illness and hardship play no part in his forgiveness. Islam teaches that hardship itself wipes away sins. God (﷾) created life and death to test people, and He tests them with fear, hunger and loss. The Prophet (ﷺ) said that no tiredness, illness, sorrow or hurt befalls a Muslim without God (﷾) wiping away some of his sins because of it. This is true even of the prick of a thorn.
“He Who created Death and Life, that He may try which of you is best in deed: and He is the Exalted in Might, Oft-Forgiving;-”
“Be sure we shall test you with something of fear and hunger, some loss in goods or lives or the fruits (of your toil), but give glad tidings to those who patiently persevere,”
Abu Sa'id al-Khudri and Abu Hurayra report that the Prophet (ﷺ) said no tiredness, illness, sorrow, sadness, hurt or distress reaches a Muslim, not even the prick of a thorn, without Allah wiping away some of his sins because of it.
God (﷾) rejected human sacrifice
Christians often say Jesus' death had to be painful and shameful because of the weight of sin. But the God (﷾) of the Bible condemned the burning of children as something "I commanded them not". Through Hosea, a Hebrew prophet of the eighth century BCE, He said He desired mercy rather than offerings. When Abraham was ready to offer his son, God (﷾) stopped him: "Lay not thine hand upon the lad".
“And they have built the high places of Tophet, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire; which I commanded them not, neither came it into my heart.”
“For I desired mercy, and not sacrifice; and the knowledge of God more than burnt offerings.”
“And he said, Lay not thine hand upon the lad, neither do thou any thing unto him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me.”
Verse by verse
Galatians 4:4 — Epistle to the Galatians, chapter 4, verse 4
“But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,”
Cited for
God sent His Son at the right moment in history, "the fulness of the time", so the timing was part of His plan.
The response
The doctrine says no sin could be removed without that death. A moment chosen thousands of years after Adam means generations lived and died before the remedy. Yet the prophets show God (﷾) forgiving people in their own day, as soon as they turned back to Him.
Where it lands — Forgiveness was given long before the cross Christians speak of, so forgiveness never waited for a cross.
Romans 3:25 — Epistle to the Romans, chapter 3, verse 25
“Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;”
Cited for
Those who lived before Jesus were forgiven in advance, because of the cross that was to come.
The response
Neither Jonah nor Jesus gives that reason. Nineveh was spared because "they turned from their evil way". The tax collector went home justified because "he that humbleth himself shall be exalted". Each passage names the sinner's own turning. Ezekiel says the sins of the man who turns back "shall not be mentioned unto him".
Where it lands — The Bible's own reasons for these pardons are repentance and humility, not a death still to come.
Romans 8:23 — Epistle to the Romans, chapter 8, verse 23
“And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.”
Cited for
Forgiven believers still suffer because the redemption of the body has not yet come.
The response
Christians give suffering other purposes, such as discipline and patience. But for many Protestants it plays no part in forgiveness, because the cross is said to have paid every debt already. That leaves the question: if the forgiven owe nothing, what is their hardship for? Islam answers it directly. The Qur'an says this life is a test. The Prophet (ﷺ) said that even the prick of a thorn wipes away some of a believer's sins.
Where it lands — In Islam a believer's hardship is part of how sins are wiped away. In the doctrine that the cross paid every debt, it has no part in forgiveness.
Perspectives
Mainstream Christian
Sin needed a perfect sacrifice, and God sent His Son at the time He had chosen. Those who lived before the cross were forgiven in view of it. Believers still suffer because the full redemption of the body is still to come.
Paul writes that God sent His Son "when the fulness of the time was come". Christians read this as God's chosen moment in history.
Rebuttal
Paul does say God (﷾) chose His moment. But if forgiveness needed the cross Christians speak of, whole generations lived and died without it. Yet the prophets show God (﷾) forgiving people in their own day, as soon as they turned back.
Paul speaks of "the remission of sins that are past, through the forbearance of God". Christians read this as forgiveness in advance for those before the cross.
Rebuttal
Paul's words can be read that way. But Ezekiel says the sins of the man who turns back "shall not be mentioned unto him". The tax collector went home justified because he humbled himself.
Paul says believers still groan while they wait for "the redemption of our body", so suffering continues until then.
Rebuttal
Christians give suffering other purposes, such as discipline and patience. But for many Protestants it plays no part in forgiveness, because the cross is said to have paid every debt. Islam gives hardship a purpose: this life is a test, and hardship wipes away sins.
The Islamic perspective
God (﷾) forgives whoever turns to Him, and always has. Adam repented and was forgiven, and every people had a messenger. The hardships of this life are a test, and through them God (﷾) wipes away the sins of the believer, as the Prophet (ﷺ) taught.
The Qur'an teaches that Adam repented and God (﷾) turned to him, with no sacrifice.
The Qur'an teaches that every people had a warner. So no people was left without a way back to God (﷾).
The Bible records people forgiven when they turned from their sins, and gives their turning as the reason.
The New Testament Gospels record Jesus declaring a man's sins forgiven long before the cross Christians speak of. They also record the crowd glorifying God (﷾), who had given such power to men.
An all-knowing God (﷾) knew from Adam's day that people would sin. A remedy delayed for thousands of years would leave whole generations without it.
An almighty God (﷾) can forgive by His will alone, so no death was required.
Through Jeremiah, God (﷾) condemned the burning of children as something He had not commanded. When Abraham was ready to offer his son, God (﷾) stopped him.
Islam teaches that hardship in this life is a test. Through it, God (﷾) wipes away a believer's sins.