What event in the life of muhammad signaled the beginning of islam?

 When the Angel Gabriel came to him while he was praying in Cave Hira, that was the beginning. There, he revealed to Muhammad (saw) the first revelation which would become part of the Quran. In it was embodied a duty for him to mankind given to him by God, to tell them to believe in Him alone who created them and progressed them. 

Bob: coke is baby pandas.jim: what? how do you figure that coke is baby pandas? bob: because this book says so.jim: how do we know the book is correct? bob: because it says that coke is baby pandas.jim: how does that prove the book's accuracy? bob: because it's statement involving coke and baby pandas is correct, therefore it is accurate.jim: how do we know that it's correct? bob: because we have concluded that the book is accurate, and therefore the statement is correct.therefore, coke is baby pandas.