The way to keep guard over our heart is immediately to expel from the mind every demon-inspired imagination—lest by dwelling on it for too long the mind is thrown headlong by the deceiver into debased and pernicious thoughts. The commandment given by God to the first man, Adam, told him to keep watch over the head of the serpent (cf. Gen. 3:15. LXX), that is, over the first inklings of the pernicious thoughts by means of which the serpent tries to creep into our souls. If we do not admit the serpents head, which is the provocation of the thought, we will not admit the rest of its body—that is, the assent to the sensual pleasure which the thought suggests—and so debase the mind towards the illicit act itself. As it is written, we should early in the morning destroy all the wicked of the earth (Ps. 101:8), distinguishing in the light of divine knowledge our sinful thoughts and then eradicating them completely from the earth—our hearts—in accordance with the teaching of the Lord. While the children of Babylon—by which I mean our wicked thoughts—are still young, we should dash them to the ground and crush them against the rock, which is Christ (cf. Ps. 137:9; I Cor. 10:4). If these thoughts grow stronger because we assent to them, we will not be able to overcome them without much pain and labor.
~St. John Cassian