In the beginning from the time of Adam and Eves children everyone only knew of one God. It was pretty obvious and there weren't many people. As the population increased people started to slowly forget the message. Their focus being turned to the life of this world and the enjoyment and of gaining respect and honour. They probably honoured some people very highly maybe due to that person being very righteous. Overtime people start having celebrations and making it a regular event. Slowly they forget why they are doing these celebrations. Newer generations may even call the honoured people demi-gods or people very close to God and then start offering prays to pass on to the real God, etc.
Some other nations maybe weren't getting rains so they prayed and nothing happened. Then someone decides to make a Rain god and some days later it rains. They say that god is true and keep on worshipping him. Some maybe decide to make a Fire/Warrior god to maybe help them in battle,etc.
If you look at most of the gods of other religions there are elaborate stories on who they are and what they did. People who worship those gods forget about what the nature of a proper God should be. All knowing, All powerful, omnipresent, no need of his creation. They make gods which they are pleased with in terms of looks and abilities and to make an interesting story.
In india a recent god has been created - google Sai Baba
https://en.wikipedia.org/wiki/Sai_Baba_of_ShirdiThis guy never claimed to be a god in my opinion but people have made him into one. Check the history out. It's an example of humans liking something and then turning it into their god. They are now inventing all sort of stories. There are even TV shows. It's fascinating seeing it happen in almost real time.
Now the children of those who worship are going to believe in what their parents believe because the love and trust them. The concepts of the religion seem great so something that looks great should be right. So the children are being mislead. Other people who look at the religion may like it and become followers.
When more new gods/religions are created people like yourself and atheists are confused and will use this as evidence that there is no god and that people just made them up, which is true up to a point. Or they will point out the contradictions in the various religions which have been man made.
I hope you get the idea. I'm not a great writer. I keep on planning to write a book but I don't think I could get my points across clearly.