How those are to be admonished who do not even begin good things, and those who do not finish them when begun.
(Admonition 35.) Differently to be admonished are they who do not even begin good things, and those who in no wise complete such as they have begun. For as to those who do not even begin good things, for them the first need is, not to build up what they may wholesomely love, but to demolish that wherein they are wrongly occupied. For they will not follow the untried things they hear of, unless they first come to feel how pernicious are the things that they have tried; since neither does one desire to be lifted up who knows not the very fact that he has fallen; nor does one who feels not the pain of a wound seek any healing remedy. First, then, it is to be shown to them how vain are the things that they love, and then at length to be carefully made known to them how profitable are the things that they let slip. Let them first see that what they love is to be shunned, and afterwards perceive without difficulty that what they shun is to be loved. For they sooner accept the things which they have not tried, if they recognize as true whatever discourse they may hear concerning the things that they have tried. So then they learn to seek true good with fullness of desire, when they have learned with certainty of judgment how vainly they have held to what was false. Let them be told, therefore, both that present good things will soon pass away from enjoyment, and also that the account to be given of them will nevertheless endure, without passing away, for vengeance; since both what pleases them is withdrawn from them now against their will, and what pains them is reserved them, also against their will, for punishment. Thus may they be wholesomely filled with alarm by the same things in which they harmfully take delight; so that when the stricken soul, in sight of the deep ruin of its fall, perceives that it has reached a precipice, it may retrace its steps backward, and, fearing what it had loved, may learn to esteem highly what it once despised.
For hence it is that it is said to Jeremiah when sent to preach, See, I have this day set you over the nations and over the kingdoms, to pluck out, and to pull down, and to destroy, and to scatter, and to build, and to plant. Because, unless he first destroyed wrong things, he could not profitably build right things; unless he plucked out of the hearts of his hearers the thorns of vain love, he would certainly plant to no purpose the words of holy preaching. Hence it is that Peter first overthrows, that he may afterwards build up, when he in no wise admonished the Jews as to what they were now to do, but reproved them for what they had done, saying, Jesus of Nazareth, a man approved of God among you by powers and wonders and signs, which God did by Him in the midst of you, as you yourselves know; Him, being delivered by the determinate counsel and foreknowledge of God, you have by the hands of wicked men crucified and slain; whom God has raised up, having loosed the pains of hell; in order, to wit, that having been thrown down by a recognition of their cruelty, they might hear the building up of holy preaching by so much the more profitably as they anxiously sought it. Whence also they immediately replied, What then shall we do, men and brethren? And it is presently said to them, Repent and be baptized, every one of you (Ibid. 37, 38). Which words of building up they would surely have despised, had they not first wholesomely become aware of the ruin of their throwing down. Hence it is that Saul, when the light from heaven shone upon him, did not hear immediately what he was to do aright, but what he had done wrong. For, when, fallen to the earth, he enquired, saying, Who are You, Lord? it was straightway replied, I am Jesus of Nazareth, whom you persecute. And when he immediately replied, Lord, what will You have me to do? it is added at once, Arise, and go into the city, and it shall be told you there what you must do. Lo, the Lord, speaking from heaven, reproved the deeds of His persecutor, and yet did not at once show him what he had to do. Lo, the whole fabric of his elation had already been thrown down and then, humble after his downfall, he sought to be built up: and when pride was thrown down, the words of building up were still kept back; to wit, that the cruel persecutor might long lie overthrown, and rise afterwards the more firmly built in good as he had fallen utterly upset from his former error. Those, then, who have not as yet begun to do any good are first to be overthrown by the hand of correction from the stiffness of their iniquity, that they may afterwards be lifted up to the state of well-doing. For this cause also we cut down the lofty timber of the forest, that we may raise it up in the roof of a building: but yet it is not placed in the fabric suddenly; in order, that is, that its vicious greenness may first be dried out: for the more the moisture thereof is exuded in the lowest, by so much the more solidly is it elevated to the topmost places.
Source: Pastoral Rule (New Advent)