Imitation of Christ

Initium omnium malarum tentationum inconstantia animi est, et parva ad Deum confidentia, quia sicut navis sine gubernaculo hinc inde a fluctibus impellitur, ita homo remissus, et suum propositum deferens varie tentatur. Ignis probat ferrum, et tentatio hominem justum. Nescimus sæpe quid possumus, sed tentatio aperit quid sumus. Vigilandum tum præcipue circa initium tentationis, quia tunc facilius hostis vincitur, si ostium mentis nullatenus intrare finitur, sed extra limen statim ut pulsaverit illi obviatur. Unde quidam dixit, Principiis obsta, sero medicina paratur. Nam primo occurrit menti simplex cogitatio, deinde fortis imaginatio, postea delectatio, et motus pravus, et assensio, sicque paulatim hostis malignus ex toto ingreditur, dum illi non resistitur in pricipio. Et quanto diutius ad resistendum quis torpuerit, tanto in se quotidie debilior fit, et hostis contra eum potentior. 
 

The beginning of all temptations to evil is instability of temper and want of trust in God; for even as a ship without a helm is tossed about by the waves, so is a man who is careless and infirm of purpose tempted, now on this side, now on that. As fire tests iron, so temptation tests the upright man. Oftentimes we know not what strength we have; but temptation reveals to us what we are. Nevertheless, we must watch, especially in the beginnings of temptation; for then is the foe the more easily mastered, when he is not suffered to enter within the mind, but is met outside the door as soon as he has knocked. Wherefore someone says,

       Check the beginnings; once thou might'st have cured,

       But now 'tis past thy skill, too long hath it endured.

For first comes to the mind the simple suggestion, then the strong imagination, afterwards pleasure, evil affection, assent. And so little by little the enemy enters in altogether, because he was not resisted at the beginning. And the longer a man delays his resistance, the weaker he grows, and the stronger the enemy grows against him.

Imitation of Christ,  I, 13:5