How can I be sure it [the Quran]
came from God?
Perhaps it should be self-evident. Have you
read it, to even see what it says? That would be a place to start. Many Muslims believe that the Qur’an’s supreme eloquence is proof that was not written by a man. The Arabs at the time of Muhammad were masters of poetry–and yet Muhammad who had
never composed a single line of poetry began to speak the Qur’an which he said had been revealed to him by God. Just like in the verse in Deuteronomy which says, “I will put my words in his mouth, and he will tell them everything I command him.” The Arabs used to critique each other’s poetry, changing words to make them more emphatic and such. But nobody ever tried to critique the Qur’an–it was too powerful for them. Many other Muslims believe that the Qur’an contains scientific insights which were unknown to the people in 7th century Arabia, which proves that it could not have been authored by God. Many other Muslims believe that the Qur’an confirms what came before it of revelation (like the messages of Moses and Jesus) and is the final message in a series. Some Muslims believe that the miracles of the Qur’an lie in the knowledge of the unseen world, things that nobody could have known except for God have been revealed in the Qur’an.
How did Muhammad convince others that it was from God?
Early on, after he had only received a few revelations, God commanded Muhammad to preach Islam to the people. So he got up on a mountain and called the people around him. He asked them, if he told them that there was an army coming from behind that mountain, would they believe him? And they said that they had never heard any lie from him. He established his credibility–they would believe him if he had said that. And then he told them that he was a warner, warning them of the punishment of the Hereafter. They rebuked him.
But the people who did believe in him–they believed because of the power of the Qur’an. He didn’t have to ‘convince’ anyone, they would hear it and the truth of it, strength of it, and beauty of it convinced them that it was from God.
How can a person put their soul on the line and say “I believe you”?
Muhammad was known as the most trustworthy among his people. The truths of the Qur’an were proven many times. There would be a prophecy, about (for example) the unlikely victor in a battle some years in the future, the Qur’an was correct. Also, when Muhammad’s family rebuked him, the Qur’an basically said that his uncle would never embrace Islam. All that uncle had to do, to prove Muhammad was wrong, was just utter fatih in Islam, even if it was a lie, but he wouldn’t do it!
The character and behavior of Muhammad was what convinced people, in addition to the Qur’an. Some people just upon seeing or hearing Muhammad would take his religion, knowing that he is not a liar.
It might be easier for one if it came from many people; but why “one man”? God has never done that before.
Of course he has. When God inspired Moses, did the Children of Israel have a book with them (like the Bible)? Or did they just believe that Moses was inspired? What about Noah? Wasn’t Noah alone when preaching to his people? Or Abraham! This is nothing new, with Muhammad.
For the converts what made you revert and stay?
Qur’an.
How can one believe this one man? this one human man? This one sinner?
It is easy to believe one man, when his statements ring true and are backed up with proof. I mean, the Qur’an appealed to me when I knew nothing about the life of Muhammad–Qur’an alone was that strong that I knew it couldn’t be the words of a liar. And it is easy to believe one man, who is known. But a variety of people who are unknown, telling contradictory stories, somehow collected into the same book? A far more dubious tale, I think. I only have to worry about what ONE MAN said, and if I can follow that. Not the sayings of dozens or hundreds of men who I don’t even know how they lived or why they wrote what they did.
I think Islam is definitely the natural and logical conclusion.

It’s so much easier to believe.