I understand your point of view. Being an atheist, you have a material understanding of the scriptures. Is the nature of your question rhetorical, are you criticizing the believe in God with it? Or do you really not know the aswer, a muslim will give? To answer your question, the only credibility of the Quran is God. Verily, if you don't believe in God, or that Muhammad was a messenger, there is not a single drop of credibility in the Holy Quran.
The interesting thing is, that even a crucifiction is relative. I want to give this thread some thoughts, that may make people understand the mismatch of the Bible and Quran version of the crucifiction. Let's look at the verse in the Quran:
And because of their saying: We slew the Messiah, Jesus son of Mary, Allah's messenger - they slew him not nor crucified him, but it appeared so unto them; and lo! those who disagree concerning it are in doubt thereof; they have no knowledge thereof save pursuit of a conjecture; they slew him not for certain.
But Allah took him up unto Himself. Allah was ever Mighty, Wise.
4:157-158
This verse is quite mysterious. They slew him not, but it appeared so unto them, what does that mean? In which way was he taken up to God? Probably, the Quran doesn't talk about Jesus body. A body can't be with God. I think, a good way to look at it, is to take the Bible. There are some interesting verses in the Bible, that explain very well the meaning of this Quran verse.
"Jesus said to her, "I am the resurrection and the life. The one who believes in me will live, even though they die"
John 11:25
"Very truly I tell you," Jesus answered, "before Abraham was born, I am!"
John 8:58
"Jesus answered them, "Destroy this temple, and I will raise it again in three days."
John 2:19
Jesus has a physical body, which can be killed. But this is not his true nature. His true nature is the eternal Logos, the Word of God.
Jesus himself implied in these 3 verses, that they didn't kill him.
How can you kill someone, who is the resurrection and the life?
If his believers live, even though they die, is it not logical, that he himself lives, even though he dies?
If he existed, before abraham, it is logical that he existed after his death. You can't kill the I AM!
This is even clearer, because Jesus raised his body up when he was dead. A dead being can't do anything. The last verse clearly stated, that Jesus was living, even though his body died.
Jesus appeared to die, but he was living the whole time, before Abraham, and after his own death. That is the meaning of the Holy Quran verse. That is why I listed some contradictions, that only appeared to be contradictions.
Exactly like "I am moving at 0 mp/h & I am moving at 1000 mp/h" are both true at the same time, depending on how you look at it.
"Jesus died & Jesus did not die" is also both true at the same time, depending on how you look at it.
A professor of Islam says the same in Wikipedia, so there are at least 2 people, who look at it like that
In regard to the interpretation of the Muslims who accept the crucifixion, Mahmoud Ayoub states:
The Qur'an is not here speaking about a man, righteous and wronged though he may be, but about the Word of God who was sent to earth and returned to God. Thus the denial of killing of Jesus is a denial of the power of men to vanquish and destroy the divine Word, which is for ever victorious.
The Holy Qur'an is very clear in confirming they did not (kill) Jesus Christ. So the crucifixion of Jesus Christ never took place. When someone is killed like in crucifixion, their body is left as evidence and that never happened in the case of Jesus Christ
