John Cassian, Conferences XI–XVII XIV.2: On grasping the knowledge of spiritual things

Whoever then would arrive at this theoretical knowledge must first pursue practical knowledge with all his might and main. For this practical knowledge can be acquired without theoretical, but theoretical cannot possibly be gained without practical. For there are certain stages, so distinct, and arranged in such a way that man’s humility may be able to mount on high; and if these follow each other in turn in the order of which we have spoken, man can attain to a height to which he could not fly, if the first step were wanting. In vain then does one strive for the vision of God, who does not shun the stains of sins: “For the spirit of God hates deception, and dwells not in a body subject to sins.”