The objection from the unchangeableness of God is answered from several passages of Scripture, wherein God promises forgiveness to sinners on their repentance. St. Ambrose also shows that mercy will be more readily accorded to such as have sinned, as it were, against their will, which he illustrates by the case of prisoners taken in war, and by language put into the mouth of the devil.
21. But they say that they make these assertions in order not to seem to make God liable to change, as He would be if He forgave those with whom He was angry. What then? Shall we reject the utterances of God and follow their opinions? But God is not to be judged by the statements of others, but by His own words. What mark of His mercy have we more ready at hand than that He Himself, through the prophet Hosea, is at once merciful as though reconciled to those whom in His anger He had threatened? For He says: “O Ephraim, what shall I do unto you, or what shall I do unto you, O Judah? Your kindness,” etc. And further on: “How shall I establish you? I will make you as Admah, and as Zeboim.” In the midst of His indignation He hesitates, as it were, with fatherly love, doubting how He can give over the wanderer to punishment; for although the Jew deserves it, God yet takes counsel with Himself. For immediately after having said, “I will make you as Admah and as Zeboim,” which cities, owing to their nearness to Sodom, suffered together in like destruction, He adds, “My heart is turned against Me, My compassion is aroused, I will not do according to the fierceness of Mine anger.”
22. Is it not evident that the Lord Jesus is angry with us when we sin in order that He may convert us through fear of His indignation? His indignation, then, is not the carrying out of vengeance, but rather the working out of forgiveness, for these are His words: “If you shall turn and lament, you shall be saved.” He waits for our lamentations here, that is, in time, that He may spare us those which shall be eternal. He waits for our tears, that He may pour forth His goodness. So in the Gospel, having pity on the tears of the widow, He raised her son. He waits for our conversion, that He may Himself restore us to grace, which would have continued with us had no fall overtaken us. But He is angry because we have by our sins incurred guilt, in order that we may be humbled; we are humbled, in order that we may be found worthy rather of pity than of punishment.
23. Jeremiah, too, may certainly teach when he says: “For the Lord will not cast off for ever; for after He has humbled, He will have compassion according to the multitude of His mercies, Who has not humbled from His whole heart nor cast off the children of men.” This passage we certainly find in the Lamentations of Jeremiah, and from it, and from what follows, we note that the Lord humbles all the prisoners of the earth under His feet, in order that we may escape His judgment. But He does not bring down the sinner even to the earth with His whole heart Who raises the poor even from the dust and the needy from the dunghill. For He brings not down with His whole heart Who reserves the intention of forgiving.
24. But if He brings not down every sinner with His whole heart, how much less does He bring down him with His whole heart who has not sinned with his whole heart! For as He said of the Jews: “This people honours Me with their lips, but their heart is far from Me,” so perhaps He may say of some of the fallen: “They denied Me with their lips, but in heart they are with Me. It was pain which overcame them, not unfaithfulness which turned them aside.” But some without cause refuse pardon to those whose faith the persecutor himself confessed up to the point of striving to overcome it by torture. They denied the Lord once, but confess Him daily; they denied Him in word, but confess Him with groans, with cries, and with tears; they confess Him with willing words, not under compulsion. They yielded, indeed, for a moment to the temptation of the devil, but even the devil afterwards departed from those whom he was unable to claim as his own. He yielded to their weeping, he yielded to their repentance, and after making them his own lost those whom he attached when they belonged to Another.
25. Is not the case such as when any one carries away captive the people of a conquered city? The captive is led away, but against his will. He must of necessity go to foreign lands, does not willingly make the journey; he takes his native land with him in his heart, and seeks an opportunity to return. What then? When any such return, does any one urge that they should not be received; with less honour indeed, but with readier will, that the enemy may have nothing with which to reproach them? If you pardon an armed man who was able to fight, do you not pardon him in whom faith alone waged the battle?
26. If we were to enquire what is the opinion of the devil concerning those who have fallen after this sort, would he not probably reply: “This people honours me with their lips, but their heart is far from me? For how can he be with me who does not depart from Christ? Without any cause do they appear to honour me who keep the doctrine of Jesus, and I thought that they would teach mine. They condemn me all the more when they forsake me after trial. Indeed Jesus is more glorified in these, when He receives them on their return to Him. All the angels rejoice, for in heaven there is greater joy over one sinner that repents, than over ninety and nine just persons who need not repentance. I am triumphed over in heaven and on earth. Christ loses nothing when they who came to me with weeping return with longing to the Church, and I am in danger even as regards my own, who will learn that in reality there is nothing here where men are led on by present rewards, but that there must be very much there where groans and tears and fasts are preferred to my feasts.”
Source: On Repentance (New Advent)