3 But the things belonging to humbleness of mind have been sufficiently spoken of; not for the value of the virtue; for no one will be able to celebrate it in accordance with its value; but for the intelligence of your love. For well do I know that even from the few things that have been said you will embrace it with much zeal. But since it is also necessary to make clear and manifest the apostolic saying which has been today read; seeming as it does to many to afford a pretext for indolence; so that some may not, providing for themselves hence a certain frigid defence, neglect their own salvation— to this let us direct our discourse.
What then is this saying? “Whether in pretence,” it says, “or in sincerity, Christ is preached.” This many wrest absolutely and just as happens, without reading what precedes and what comes after it; but having cut it off from the sequence of the remaining members, to the destruction of their own soul they put it forward to the more indolent. For attempting to seduce them from the sound faith; then seeing them afraid and trembling; on the ground of its not being without danger to do this, and desiring to relieve their fears, they bring forward this apostolic declaration, saying, Paul conceded this, by saying, “Whether in pretence or in sincerity, let Christ be proclaimed.”
But these things are not (true), they are not. For in the first place he did not say “let him be proclaimed,” but “he is proclaimed,” and the difference between this and that is wide. For the saying “let him be proclaimed” belongs to a lawgiver; but the saying “he is proclaimed” to one announcing the event. For that Paul does not ordain a law that there should be heresies, but draws away all who attended to him, hear what he says, “If any one preaches to you a gospel besides what you have received, let him be anathema, were it even I, were it even an angel from the heavens.” Now he would not have anathematized both himself and an angel, if he had known the act to be without danger.
And again— “I am jealous of you with a jealousy of God,” he says; “for I have betrothed you to one husband a chaste virgin: and fear lest at some time, as the serpent beguiled Eve by his wiliness, so your thoughts should be corrupted from the singleness that is towards Christ.” See, he both set down singleness, and granted no allowance. For if there were allowance, there was no danger: and if there was no danger Paul would not have feared: and Christ would not also have commanded that the tares should be burned up, if it were a thing indifferent to attend to this one or that or another: or to all indiscriminately.
Source: Homily Concerning "Lowliness of Mind" (New Advent)