How are the gods forgotten?
I haven't played the game, but I am up to date with the anime. So I will use that as my source.

By what we have been told so far the gods weren't forgotten. They were simply neglected. The society was aware about their existence, but simply didn't worship them anymore. It most likely became a hassle as they became more and more independent from the "gods".

Most people were aware from stories told by the elder people that there was a temple, but they didn't care enough to actually check it out.