Here's the full quote:
"To sell your soul is the easiest thing in the world. That's what everybody does every hour of his life. If I asked you to keep your soul - would you understand why that's much harder?"
Here, "selling your soul" means being a second-hander: letting others determine your values and your convictions. It's the way Peter Keating lives his life. In contrast, "keeping your soul" means being independent, a first-hander: taking the responsibility of thinking and deciding for yourself, choosing your own values.
Roark is saying that being a second-hander is defaulting on that responsibility, by dropping the effort of thinking. To keep your soul, to be independent, requires thought and effort.
answered Feb 13 '11 at 03:17