❧ Foundations
Islam and Christianity agree on more than most realize — and disagree on the questions that matter most. The crucifixion, the Quran's Jesus, the Trinity, Muhammad's prophethood, and the textual history of the Quran examined directly.
Islam and Christianity are not as far apart as their adherents sometimes assume — and not as close as their diplomats sometimes pretend. They share a remarkable amount of common ground: one God who created the universe, Abraham as the father of faith, Moses as lawgiver and prophet, a coming day of judgment, bodily resurrection, heaven and hell, and — crucially — a deep reverence for Jesus of Nazareth. Islam does not dismiss Jesus. It honors him. The Quran calls him the Messiah, born of a virgin, sinless, a worker of miracles, the Word of God, a Spirit from God, and one who will return at the end of history. By any measure, this is an extraordinarily elevated view of a figure that Islam nevertheless insists was not divine.
This is precisely the problem. Because the disagreements between Islam and Christianity are not peripheral. They are not disputes about liturgy or tradition or secondary doctrine. They cut directly to the identity of Jesus, the meaning of his death, the nature of God, and the reliability of the documents that Christians and Muslims both claim to respect. These disagreements are mutually exclusive. Both religions cannot be true simultaneously. Either Jesus died on the cross and rose from the dead — in which case Christianity's central claim is vindicated and Islam's central denial is false — or Jesus did not die on the cross — in which case every Gospel, every epistle, every eyewitness account, and every non-Christian historical source that mentions his death is either fabricated or deceived. There is no comfortable middle ground.
This Foundation examines those disagreements directly. It does so with respect for Muslim people and Muslim scholarship — Islam has produced extraordinary thinkers, scientists, poets, and saints across fourteen centuries, and the sincere faith of individual Muslims is not in question here. What is in question is whether Islam's specific claims about Jesus, about the Bible, about Muhammad, and about God are true. That question deserves the same honest, evidence-based examination that the claims of Christianity received in the Why Christianity Foundation.
Before examining where Islam goes wrong, it is worth being precise about where it goes right — because acknowledging this honestly is the only foundation for a productive examination.
Islam gets the most important metaphysical fact right: there is one God who created the universe, who is transcendent, all-knowing, all-powerful, and morally serious. The Quran's condemnation of idolatry is uncompromising and correct. Its insistence that God is not to be confused with his creation — that the creature should not be worshipped instead of the Creator — is a genuine theological contribution that preserved monotheism in the Arabian Peninsula when it might otherwise have been lost to polytheism. Islam's emphasis on prayer, on gratitude, on submission to God's will, on care for the poor, on the equality of human beings before God — these are real goods. They are not the exclusive property of Islam, but Islam carries them genuinely.
Islam also preserves significant historical continuity with the biblical tradition. It affirms that God revealed himself to Adam, Noah, Abraham, Isaac, Ishmael, Jacob, Moses, David, and Jesus before Muhammad. It affirms the reality of angels, of divine judgment, of the afterlife. It affirms that Jesus was born of a virgin, performed miracles, and will return. On these points, Islam and Christianity stand together against the secular modern consensus that dismisses all of it.
The disagreements are therefore not between a religion that takes God seriously and one that does not. They are between two religions that take God with the utmost seriousness — and that disagree about the most important things he has done.
The disagreements between Islam and Christianity are not vague theological differences susceptible to harmonization. They are specific, concrete, and mutually exclusive.
On the crucifixion: Christianity says Jesus was crucified, died, and rose from the dead — and that this death accomplished the atonement of human sin. Islam says Jesus was not crucified — that it only appeared so, that someone else was substituted, and that God raised Jesus directly to heaven without his dying. Both cannot be true.
On the divine identity of Jesus: Christianity says Jesus is the eternal Son of God — fully divine and fully human, the second person of the Trinity who took on human flesh. Islam says this is the worst possible blasphemy (shirk): associating a partner with God. Surah 5:72 says: "Whoever associates others with Allah — Allah has forbidden him paradise, and his refuge is the Fire." Both cannot be true.
On the nature of God: Christianity says God is triune — one being, three persons, Father, Son, and Holy Spirit. Islam says God is absolutely unitary — one being, one person, and to say otherwise is the unpardonable sin. Both cannot be true.
On the reliability of the Bible: Christianity says the Old and New Testaments are the preserved Word of God, reliably transmitted, and the standard against which all subsequent claims must be measured. Islam says the Bible has been corrupted (tahrif) by Jews and Christians — and that the Quran is the final, uncorrupted revelation that supersedes it. Both cannot be true.
On salvation: Christianity says human beings are fallen, that sin creates a moral debt that human effort cannot repay, and that God himself paid that debt in the person of Jesus Christ — so that salvation is received by grace through faith, not earned by works. Islam says sin is primarily a matter of individual failures that can be addressed by repentance and obedience — that each person's good and bad deeds will be weighed on the Day of Judgment, and that God's mercy may tip the scales. Both cannot be true.
These are not small differences. They are the largest possible differences. And since they concern historical events that either happened or did not, and theological claims that are either coherent or not, they can be examined.
This Foundation does not approach Islam as an enemy to be defeated. It approaches it as a serious system of truth claims that deserves serious examination. The method throughout is the same method applied to Christianity in the first Foundation: follow the evidence, present the strongest version of each argument, give the strongest responses their due, and ask what conclusion the evidence most honestly supports.
Several things will be found throughout:
The evidence for the crucifixion of Jesus is, by the standard of ancient historiography, among the most secure facts in the ancient world — attested by Roman historians, Jewish sources, and the unanimous testimony of people who had every reason to know. Islam's denial of the crucifixion, made 600 years after the event, has no historical support that does not derive from the Quran itself.
The Quran contains specific historical errors — confusions of persons separated by centuries, anachronisms, and misidentifications — that are inconsistent with the claim of direct divine dictation by an omniscient God.
The Quran's understanding of the Trinity is not the Trinity that Christianity has ever taught. Its refutation is therefore a refutation of a position that orthodox Christianity has never held.
The theological framework of Islam — its account of sin, salvation, and the nature of God — leaves unresolved problems that the Christian framework resolves. This is not merely a matter of preference; it is a question of internal coherence.
None of this requires contempt for Muslim people. It requires honesty about what the evidence shows.