Hang on, that's different from what you said earlier. What you were criticising then was that there could be multiple ways to God, not that all religions lead to God. Nobody has proposed that all religions do so, and that is not what the OP asked either. Don't move the goalposts.
The proposition is that there may be multiple ways to God. As for where this idea comes from, it comes from people's heads. There are some who think for themselves, based on what they observe in the world around them. Not everyone has to be told what to think by reading a text.