We must not imagine that anyone slips and comes to grief by a sudden fall, but that he falls by a hopeless collapse either from being deceived by beginning his training badly, or from the good qualities of his soul failing through a long course of carelessness of mind, and so his faults gaining ground upon him little by little.
Saint John Cassian, Conference with Abbot Theodore, ch. xvii
If, as scripture teaches, everything involuntary has its cause in what is voluntary, man has no greater enemy than himself.
St. Mark the Ascetic, Philokalia, Volume I, page 133