Where I'm from it's not a spiritual thing, either; from what I understand, however, some humans believe that one's soul bears the results of one's morality. Basically, the better a person they are, the purer one's soul will be; I'm reasonably sure, in those circumstances, that one's soul is seen as a gift from a God-figure of some sort, hence the faith aspect - it leads to those that don't believe in God to discard the notion of a soul entirely, as there's no God to give them one in the first place.
In my world, that isn't even taken into consideration by most - the soul is very definitely an actual object, something that can be understood and possibly physically manipulated, and while I've been taught that there is...not God, per se, but a very specific being that keeps everything balanced, as long as we aren't completely defying nature or creating abominations or otherwise horribly unbalancing the forces of the world, we can do whatever we want regardless of the morality of it.
Of course, there are some that believe otherwise in Amestris, but they're in the minority.
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In my world, that isn't even taken into consideration by most - the soul is very definitely an actual object, something that can be understood and possibly physically manipulated, and while I've been taught that there is...not God, per se, but a very specific being that keeps everything balanced, as long as we aren't completely defying nature or creating abominations or otherwise horribly unbalancing the forces of the world, we can do whatever we want regardless of the morality of it.
Of course, there are some that believe otherwise in Amestris, but they're in the minority.