3 Yet you are not the sole author of the evil, but there is also another most wicked prompter, the devil. He indeed suggests, but does not get the mastery by force over those who do not consent. Therefore says the Preacher, If the spirit of him that has power rise up against you, quit not your place. Shut your door, and put him far from you, and he shall not hurt you. But if you indifferently admit the thought of lust, it strikes root in you by its suggestions, and enthrals your mind, and drags you down into a pit of evils.
But perhaps you say, I am a believer, and lust does not gain the ascendant over me, even if I think upon it frequently. Do you not know that a root breaks even a rock by long persistence? Admit not the seed, since it will rend your faith asunder: tear out the evil by the root before it blossom, lest from being careless at the beginning thou have afterwards to seek for axes and fire. When your eyes begin to be diseased, get them cured in good time, lest you become blind, and then have to seek the physician.
Source: Catechetical Lectures (New Advent)