7 Let us not then be confident, who stand, but let us say to ourselves, “Let him that thinks he stands take heed lest he fall;” neither let us who are fallen despair, but let us say to ourselves, “He that falls, does he not arise?” For many even who have mounted to the very summit of Heaven, and have shown forth all austerity, and had made their abode in the deserts, nor saw any woman so much as in a dream; having become a little remiss, have been tripped up, and have come unto the very gulf of wickedness. While others again from thence have gone up to Heaven, and from the stage and orchestra have passed over unto the discipline of angels, and have displayed so great virtue, as to drive away devils, and to work many other such miracles. And of these examples both the Scriptures are full, and our life is also full. Even whoremongers and effeminate persons stop the mouths of the Manichæans, who say that wickedness is immoveable, enrolling themselves on the devil's side, and weakening the hands of them that would wish to be in earnest, and overturning all our life.
For they who inculcate these things, not only injure men as to the future, but here also turn all things upside down, for their own part at least. Because when will any regard virtue, from among those that are living in wickedness, so long as he accounts his return that way, and his change for the better, a thing impossible? For if now, when both laws exist, and penalties are threatened, and there is common opinion to recall the ordinary sort, and hell is looked for, and a kingdom promised, and wrong things reproached, and the good praised; hardly do any choose the labors that are to be undergone for virtue's sake: should you take away all these things, what is there to hinder ruin and corruption universal?
Knowing therefore the devil's craft, and that as well the lawgivers of the Gentiles as the oracles of God, and the reasonings of nature, and the common opinion of all men, yea barbarians, and Scythians, and Thracians, and generally all, are directly opposed both to these, and to such as strive to enact the doctrines of fate: let us be sober, beloved, and bidding farewell to all those, let us travel along the narrow way, being both confident and in fear: in fear because of the precipices on either side, confident because of Jesus our guide. Let us travel on, sober and wakeful. For though but for a little while one slumber, he is swept away quickly.
Source: Homilies on the Gospel of St. Matthew (New Advent)