I made the terrible mistake of entering upon the Christian life as if it were merely the natural life invested with a kind of supernatural mode by grace. I thought that all I had to do was to continue living as I had lived before, thinking and acting as I did before, with the one exception of avoiding mortal sin. It never occurred to me that if I continued to live as I had lived before, I would be simply incapable of avoiding mortal sin.

source: Thomas Merton, Seven Storey Mountain 280 tags: Habit, Sin