http://blog.ayjay.org/augustine-and-pelagius/

It makes a rigorous asceticism the only true Christian life — as [Peter] Brown points out, “Pelagius wanted every Christian to be a monk” — and condemns even the most determined ascetic to constant self-scrutiny, a kind of self-scrutiny that can never yield a clear acquittal. You might have missed something; and in any case you could sin in the next five minutes and watch your whole house of cards crash down.