But the crucifixion of Jesus (specifically) is a historical fact, confirmed by many other sources, not some random claim 600 years later like Muhammad said, which contradicts what people back then actually reported.Muslims don't deny that a crucifixion took place; only that it was Jesus who was crucified. It is not true that the Bible is the exact same as primitive versions. 1 John 5:7-8 is an example of a later addition to the Bible - scholars agree it does not appear in the earliest Greek manuscripts and that it first shows up in Latin manuscripts centuries later, likely to support trinitarian beliefs.
Yes, the Bible isn’t exactly the same as the earliest manuscripts, that’s true, but saying the central message changed is absurd. The Bible has over 66,000 cross-references and the text is internally consistent from start to finish. A text edited carelessly wouldn’t have that.
Most manuscripts are about 99% the same, even though they’re independent, and only 1% is different. And that 1%? Most of it, like 99% of the 1%, is just grammar or spelling differences, like “Jesus” or “Iesous”, “the” or “a”, which don’t affect meaning. Then 0.9% are minor differences that also don’t change the message. The so-called “problem” is in the last 0.1%—things like the story of the woman caught in adultery (John 7:53–8:11) or the long ending of Mark (Mark 16:9–20). But even those don’t affect the central message, and much of it is confirmed in other Gospels anyway.
Sources: Codex Sinaiticus, Codex Vaticanus, Bruce Metzger The Text of the New Testament.