St. Ambrose proceeds with the proof of the divine mercy, and shows by the testimony of the Gospels that it prevails over severity, and he adduces the instance of athletes to show that of those who have denied Christ before men, all are not to be esteemed alike.
15. Although what has been said sufficiently shows how inclined the Lord Jesus is to mercy, let Him further instruct us with His own words, when He would arm us against the assaults of persecution. “Fear not,” He says, “those who kill the body, but cannot kill the soul, but rather fear Him Who can cast both body and soul into hell.” And farther on: “Every one, therefore, who shall confess Me before men, him will I also confess before My Father, Who is in heaven, but he who shall deny Me before men, him will I also deny before My Father, Who is in heaven.”
16. Where He says that He will confess, He will confess “every one.” Where He speaks of denying, He does not speak of denying “every one.” For, whereas in the former clause He says, “Every one who shall confess Me, him will I confess,” we should expect that in the following clause He would also say, “Every one who shall deny Me.” But in order that He might not appear to deny every one, He concludes: “But he who shall deny Me before men, him will I also deny.” He promises favour to every one, but He does not threaten the penalty to every one. He makes more of that which is merciful. He makes less of what is penal.
17. And this is written not only in that book of the Gospel of the Lord Jesus, which is written according to Matthew, but it is also to be read in that which we have according to Luke, that we might know that neither had thus related the saying by chance.
18. We have said that it is thus written. Let us now consider the meaning. “Every one,” He says, “who shall confess Me,” that is to say, of whatever age, of whatever condition he may be, who shall confess Me, he shall have Me as the Rewarder of his confession. Whereas the expression is, “every one,” no one who shall confess is excluded from the reward. But it is not said in like manner, “Every one who shall deny shall be denied,” for it is possible that a man overcome by torture may deny God in word, and yet worship Him in his heart.
19. Is the case the same with him who denies voluntarily, and with him whom torture, not his own will, has led to denial? How unfit were it, since with men credit is given for endurance in a struggle, that one should assert that it had no value with God! For often in this world's athletic contests the public crown together with the victors even the vanquished whose conduct has been approved, especially if perchance they have seen that they lost the victory by some trick or fraud. And shall Christ suffer His athletes, whom He has seen to yield for a moment to severe torments, to remain without forgiveness?
20. Shall not He take account of their toil, Who will not cast off for ever even those whom He casts off? For David says: “God will not cast off for ever,” and in opposition to this shall we listen to heresy asserting, “He does cast off for ever”? David says: “God will not for ever cut off His mercy from generation to generation, nor will He forget to be merciful.” This is the prophet's declaration, and there are those who would maintain a forgetfulness of mercy on God's part.
Source: On Repentance (New Advent)