Every use in the Bible that I can find has "soul" referring to desires, ambitions and vulnerabilities. Spirit, however, is invulnerable and craves not the things of this world.
I suspect that "soul" refers to Ego -- that which hungers to be "first," but which will end up being last.
I had this all wrong for most of my life. Perhaps I still have more to learn, but this is my current view on the topic.
Spirit is the true self. When the Bible says that the only unforgivable sin is that of denying the Holy Spirit, the reason becomes obvious when you see that this spirit is the one (true self) which needs rescue. If you deny the object of rescue, then rescue (forgiveness) can never happen.
This is why atheists are condemned. They condemn themselves. It is not for us to judge them, but to lament for them and pray for them. In their madness, they are spitting in the face of the rescuer. They are cutting the rope which is meant to lift them up.