Post History

My background is in the reformed tradition, which necessarily impacts my response, but I would go all the way back to Augustine to find historic Christian thought on the subject. Augustine taught that there were four states of man's nature: 
* Man pre-fall, able to either sin or not sin (_posse peccare, posse non peccare_)
* Man post-fall, unable to refrain from sin (_non posse non peccare_)
* Regenerate man, able not to sin (_posse non peccare_), and 
* Glorified man, unable to sin (_non posse peccare_). 

The driving factor for whether man can sin depends on whence the impulse to sin originates. Man sins when his will is drawn to evil; when he is tempted (Jonathan Edwards expounds on this extensively in _Freedom of the Will_). Adam and Eve were subjected to temptation in the Garden of Eden and failed as moral agents. Each of us is tempted daily, entrapped by our sinful natures, the world, and the devil, and we fail. Once glorified and as residents of Heaven, however, we will no longer be exposed to temptation from those sources; the devil will be thrown into the lake of fire, and we will be clothed in fine linen. As a result, our free will is unencumbered to do righteousness.