The word religion comes from the Latin “religare” and means: “to bring together” or, more common today, “to unite”, to be one with god. The idea of a religion is to teach you how to get to that state. It is a road map to enlightenment. Throughout history there have been people who teach us how to reach the highest possible form of being. If you understand that, you become the perfect human being. That is the basic function of a religion on a personal level.
The path to enlightenment is a difficult task to realize, especially if you read only one book. Everyone has their own opinion on how to interpret the words of books. Words are always a translation of a translation.
There is a deeper complexity: suppose a supreme being exists and sends knowledge to earth using people. I do not know how, I am open-minded, but skeptical. The people that god sends, choose their words to explain what god wants to communicate. They do the first interpretation. I think we can agree that there is only one god, if it exists. The words and concepts, that those people chose to explain the revealed knowledge, are linked to their language, culture and the period of history in which they were pronounced.
And, by “choosing” I mean, not that they had a choice, but the words they used to be able to translate the knowledge they had in the best possible way for others to understand as well.