1 Corinthians 1:9
But you say, “I am not of this sort in my bodily frame, or my turn of mind.” That is for want, not of power, but of will. For thus I prove that all have a certain aptness towards virtue: That which a man cannot do, neither will he be able to do though necessity be laid upon him; but, if, necessity being laid upon him, he is able, he that leaves it undone, leaves it undone out of choice. The kind of thing I mean is this: to fly up and be borne towards heaven, having a heavy body, is even simply impossible. What then, if a king should command one to do this, and threaten death, saying, “Those men who do not fly, I decree that they lose their heads, or be burnt, or some other such punishment:” would any one obey him? Surely not. For nature is not capable of it. But if in the case of chastity this same thing were done, and he were to lay down laws that the unclean should be punished, be burnt, be scourged, should suffer the extremity of torture, would not many obey the law? “No” you will say: “for there is appointed, even now, a law forbidding to commit adultery and all do not obey it.” Not because the fear looses its power, but because the greater part expect to be unobserved. So that if when they were on the point of committing an unclean action the legislator and the judge came before them, the fear would be strong enough to cast out the lust. Nay, were I to apply another kind of force inferior to this; were I to take the man and remove him from the beloved person, and shut him up close in chains, he will be able to bear it, without suffering any great harm. Let us not say then that such an one is by nature evil: for if a man were by nature good, he could never at any time become evil; and if he were by nature evil, he could never be good. But now we see that changes take place rapidly, and that men quickly shift from this side to the other, and from that fall back again into this. And these things we may see not in the Scriptures only, for instance, that publicans have become apostles; and disciples, traitors; and harlots, chaste; and robbers, men of good repute; and magicians have worshipped; and ungodly men passed over unto godliness, both in the New Testament and in the Old; but even every day a man may see many such things occurring. Now if things were natural, they could not change. For so we, being by nature susceptible, could never by any exertions become void of feeling. For that which is whatever it is by nature, can never fall away from such its natural condition. No one, for example, ever changed from sleeping to not sleeping: no one from a state of corruption unto incorruption: no one from hunger to the perpetual absence of that sensation. Wherefore neither are these things matters of accusation, nor do we reproach ourselves for them; nor ever did any one, meaning to blame another, say to him, “O thou, corruptible and subject to passion:” but either adultery or fornication, or something of that kind, we always lay to the charge of those who are responsible; and we bring them before judges, who blame and punish, and in the contrary cases award honors.
11. Since then both from our conduct towards one another, and from others' conduct to us when judged, and from the things about which we have written laws, and from the things wherein we condemn ourselves, though there be no one to accuse us; and from the instances of our becoming worse through indolence, and better through fear; and from the cases wherein we see others doing well and arriving at the height of self-command, (φιλοσοφίας) it is quite clear that we also have it in our power to do well: why do we, the most part, deceive ourselves in vain with heartless pretexts and excuses, bringing not only no pardon, but even punishment intolerable? When we ought to keep before our eyes that fearful day, and to give heed to virtue; and after a little labor, obtain the incorruptible crowns? For these words will be no defence to us; rather our fellow-servants, and those who have practised the contrary virtues, will condemn all who continue in sin: the cruel man will be condemned by the merciful; the evil, by the good; the fierce, by the gentle; the grudging, by the courteous; the vain-glorious, by the self-denying; the indolent, by the serious; the intemperate, by the sober-minded. Thus will God pass judgment upon us, and will set in their place both companies; on one bestowing praise, on the other punishment. But God forbid that any of those present should be among the punished and dishonored, but rather among those who are crowned and the winners of the kingdom. Which may God grant us all to obtain through the grace and loving-kindness of our Lord Jesus Christ; with Whom unto the Father and the Holy Ghost be glory, power, honor, now and ever, and unto everlasting ages. Amen.
Source: Homilies on First Corinthians (New Advent)