There was no Islam before Mohammed, but it was a conglomeration of the peaceful aspects of several religions that impressed him in his early years. Islam changed when Mohammed married a rich woman which gave him power through a hired army. The power corrupted him and his cult from then on.
You need to go further than your resource that is stating this...just for a start the religions were pagan Gods, and even today Islam uses the crescent moon symbol of a pagan God.
That way he started with a small following.