The Conferences Of John Cassian by John Cassian
Chapter XX. From whom we can learn the destruction of sin and perfection of goodness.
Lastly if you would like to investigate more thoroughly whether it is possible for human nature to attain sinlessness, from whom can we more clearly learn this than from those who |have crucified the flesh with its faults and lusts,| and to whom |the world is really crucified|? Who though they have not only utterly eradicated all faults from their hearts, but also are trying to shut out even the thought and recollection of sin, yet still day after day faithfully maintain that they cannot even for a single hour be free from spot of sin.