ambrose_of_milan[St Ambrose is addressing the question of those who deny Christ in the face of persecution.]

Although what has been said sufficiently shows how inclined the Lord Jesus is to mercy, let Him further instruct us with His own words…:

“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” (Matt. 10:32, 33).

Where He says that He will confess, He will confess “everyone.” Where He speaks of denying, He does not speak of denying “everyone.”

For, whereas in the former clause He says “everyone who shall confess Me, him will I confess,” we should expect that in the following clause He would also say “everyone 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 everyone, but He does not threaten the penalty to everyone. He makes more of that which is merciful. He makes less of what is penal.

[…]  “Everyone,” 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. As the expression is “everyone,” no one who shall confess is excluded from the reward.

But it is not said in like manner, “Everyone 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.

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?  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 forever” (Ps. 76:7).

[…] David says: “God will not forever cut off His mercy from generation to generation, nor will He forget to be merciful” (Ps. 76:8, 9).

Ambrose of Milan (c. 337-397): On Repentance, book 1, chapter 4, 15-20.