I'm not sure if you can be certain that 'Heaven' is just a notion, just because it can't be physically or tangibly proven at this point in time doesn't prove it's 100% untrue with zero probability of being real but we're all designed to have our own opinions and thats a good thing imo.
When it comes to Churches have you ever thought that many moons ago they might have been taken over or maybe even started by bad actors and then used in ways like you describe?
I don't think Jesus built any Church building, he just went around spreading the word wherever people listened from what i understand, i don't see any need to go to a Church to be able to believe in God or Jesus and if God is the word and Jesus is the light then should we limit our thinking to just the physical world when trying to understand the Bible?
