The argument has force against the idea that early Christianity was judged by a finished New Testament from day one. But it does not prove that the earliest believers had no way to know anything true about Jesus; it proves that their certainty depended heavily on living transmission, community authority, and later canon-making.
Why the question is a serious one
The strongest form of the objection is this: if the four written Gospels were not all composed, copied, circulated, and recognized everywhere in the first generation, then how did early Christians test what was authentic? A believer in Antioch, Corinth, or Rome may have had some letters, some oral reports, public preaching, and the Hebrew scriptures, but not a complete New Testament bound in one book.
So if later Christians say, "Our faith is based on the Bible," the question presses: which Bible, at what date, and who decided? Before a settled canon, competing groups also claimed to preserve the true meaning of Jesus - Jewish-Christian groups, Pauline churches, Gnostics, Marcionites, and others. The issue is not whether early Christians were sincere. It is whether sincerity plus partial documents can establish doctrinal authenticity with certainty.
How early Christians actually judged matters
The best historical reply is that early Christians did not rely on a completed Gospel collection at first. They relied on several things together: the preaching of those believed to be apostles, memorized teachings of Jesus, communal worship, baptismal confessions, the Jewish scriptures, letters from recognized leaders, and the authority of churches believed to have apostolic roots.
This is a real answer, not a weak one. Most religious communities begin through teaching before books are gathered. Even the Quran was recited, memorized, written, and then compiled as a mushaf. Oral transmission is not automatically unreliable. A living community can preserve core teachings, especially when they are repeated in worship and taught publicly.
But there is a difference. In Islam, the Quran presents itself as the very revelation given to the Prophet Muhammad (PBUH), recited in known wording and preserved by mass memorization and writing. The Gospels are not, in Christian scholarship itself, transcripts of a book revealed to Jesus. They are later narratives about him, written in Greek, by believers, after a period of preaching and interpretation.
What the Quran says about the Gospel
The Quran affirms that Jesus was given revelation, guidance, and light. It does not say that Matthew, Mark, Luke, and John are identical to the Injil revealed to Jesus.
And We sent, following in their footsteps, Jesus, the son of Mary, confirming that which came before him in the Torah; and We gave him the Gospel, in which was guidance and light.
Quran 5:46
It also says the Quran came as a criterion over previous scripture:
And We have revealed to you the Book in truth, confirming that which preceded it of the Scripture and as a criterion over it.
Quran 5:48
That is the balanced position: there was genuine guidance with Jesus, and traces of truth remain, but later transmission and interpretation are not treated as immune from human alteration, selection, or theological development.
What this argument proves - and what it does not
It proves that early Christianity cannot honestly be described as a religion that began with a complete, universally recognized New Testament. It also weakens claims that doctrine can be proven from "the Bible alone" without depending on church tradition and canon decisions.
It does not prove that every early Christian belief was false, or that no authentic teaching of Jesus survived. A Muslim should not argue carelessly. The earliest followers of Jesus certainly had access to some true teaching, and the Quran honors the sincere disciples who supported him.
What the argument does establish is more modest and more important: the later Christian canon and doctrines were filtered through decades of oral preaching, communal authority, disputes, and selection. That makes the Islamic claim reasonable: God sent the Quran not as a vague replacement, but as a preserved criterion to confirm what remained true and correct what people disputed.
So the question is not, "Did early Christians know anything?" They did. The sharper question is, "Can later Christianity guarantee that its canon and creed perfectly preserve the revelation given to Jesus?" From an Islamic view, that guarantee belongs to revelation protected by God, not to a canon recognized only after a long human process.