There are many straight and narrow paths which exclude the supernatural as motivation.
Spirituality and deep-seated moral compasses are not the exclusive domain of theists. Some of the most spiritual people I know are atheists. All of the least spiritual people I know are rabid Christians. Both facts are certainly a result of my particular circumstances and aren't meant to generalize to either group.
> On the contrary, it's really pointless to try hard to be a good person if getting to heaven is the concern.
Many seminary lectures have been filled by the intellectual quagmires that grow out of "let grace abound".
> I don't know who teaches this stuff people have in their heads
IME growing up in the church, mostly American Protestants. And I don't only mean that in a coy way. Many American Protestant churches do not require their clergy attend seminary. The result is predictable -- some excellent diamonds in the rough giving competent sermons, surrounded by a whole bunch of crazy rants vaguely grounded in some words that do indeed come from the bible.