Here is how I believe the different religions started:

When an ancient people had spiritual encounters, they might have interpreted them differently than someone else and sense most people don't have these kinds of experiences they would believe the interpretation of someone else was "true" and that others were "false" causing religions to form. Because many religions look for converts, they would often have to translate their religious texts and beliefs to the languages spoken by the people the wished to convert. Through mistranslation stories would change a little over time resulting in a slight variation in belief from the original religion and over enough time this could have resulted in entirely new religions