The Quran was revealed to Muhammad pbuh who was an illiterate man who claimed to be a prophet just like Abraham, Moses and Jesus.
The question is how do we know he was telling the truth? He was either Lying, deluded or telling the truth:
Before he claimed to be a prophet, he was nicknamed “”Al Amin” the trust worthy one as he was never known to lie. People would lie to get material gain however he was offered wealth, position and women to leave his message but he rejected this and instead lived a life of struggle.
The Prophet Mohammad Decision
He might think he is a prophet but is not.However someone who is deluded could bring about a miraculous book like the Quran. The Quran could not be produced by a human so for it to be revealed to Mohammad shows that he was not deluded.
So therefore when we look at the life and character of Muhammad pbuh we see he was telling the truth and that he is what he claims which is a prophet of God.