I had dinner with an old friend last night, and he told me his "latest theory" on God:
"To say that someone else exists in the world is already religious."
I thought it was utterly brilliant. To admit that someone else exists, with feelings, thoughts, dreams and hopes is to make an act of faith and to go outside of the lonely prison of yourself. You'll never really know the inside of another person; but to admit that it is there, and is as central to the world as your own inner self, well, that's a spiritual assertion made by faith.
If then you start to manage your life around the admission that other people exist apart from your own needs and desires and gratification, then you've moved from faith to practice.
Sin is simply the failure to see others apart from our own terms and needs and desires. They're just props in the world to help us with our need for recognition, praise, pleasure and security. When it's really bad it gets called ministry: other people exist to serve our need to help them or save them. This kind of selfishness is the worst because it masquerades as altruism and helpfulness.
To admit that you yourself aren't the center of the world, in spite of all appearances and suggestions to the contrary in your own mind and heart, that's the beginning of spirituality. To admit that there is an "other" is the beginning of admitting that there is Otherness Itself, the mystery that we clumsily call "God."