To the argument of the Novatians, that they only deny forgiveness in the case of greater sins, St. Ambrose replies, that this is also an offense against God, Who gave the power to forgive all sins, but that of course a more severe penance must follow in case of graver sins. He points out likewise that this distinction as to the gravity of sins assigns, as it were, severity to God, Whose mercy in the Incarnation is overlooked by the Novatians.
10. But they say that, with the exception of graver sins, they grant forgiveness to those of less weight. This is not the teaching of your father, Novatian, who thought that no one should be admitted to penance, considering that what he was unable to loose he would not bind, lest by binding he should inspire the hope that he would loose. So that your father is condemned by your own sentence, you who make a distinction between sins, some of which you consider that you can loose, and others which you consider to be without remedy. But God does not make a distinction, Who has promised His mercy to all, and granted to His priests the power of loosing without any exception. But he who has heaped up sin must also increase his penitence. For greater sins are washed away by greater weeping. So neither is Novatian justified, who excluded all from pardon; nor are you, who imitate and, at the same time, condemn him, for you diminish zeal for penance where it ought to be increased, since the mercy of Christ has taught us that graver sins must be made good by greater efforts.
11. And what perversity it is to claim for yourselves what can be forgiven, and, as you say, to reserve to God what cannot be forgiven. This would be to reserve to oneself the cases for mercy, to God those for severity. And what as to that saying: “Let God be true but every man a liar, as it is written, That You might be justified in Your words, and overcome when You are judged”? In order, then, that we may recognize that the God of mercy is rather prone to indulgence than to severity, it is said: “I desire mercy rather than sacrifice.” How, then, can your sacrifice, who refuse mercy, be acceptable to God, since He says that He wills not the death of a sinner, but his correction?
12. Interpreting which truth, the Apostle says: “For God, sending His own Son in the likeness of sinful flesh, and for sin condemned sin in the flesh, that the righteousness of the Law might be fulfilled in us.” He does not say “in the likeness of flesh,” for Christ took on Himself the reality not the likeness of flesh; nor does He say in the likeness of sin, for He did no sin, but was made sin for us. Yet He came “in the likeness of sinful flesh;” that is, He took on Him the likeness of sinful flesh, the likeness, because it is written: “He is man, and who shall know Him?” He was man in the flesh, according to His human nature, that He might be recognized, but in power was above man, that He might not be recognized, so He has our flesh, but has not the failings of this flesh.
13. For He was not begotten, as is every man, by intercourse between male and female, but born of the Holy Spirit and of the Virgin; He received a stainless body, which not only no sins polluted, but which neither the generation nor the conception had been stained by any admixture of defilement. For we men are all born under sin, and our very origin is in evil, as we read in the words of David: “For lo, I was conceived in wickedness, and in sin did my mother bring me forth.” Therefore the flesh of Paul was a body of death, as he himself says: “Who shall deliver me from the body of this death?” But the flesh of Christ condemned sin, which He felt not at His birth, and crucified by His death, so that in our flesh there might be justification through grace, in which before there had been pollution by guilt.
14. What, then, shall we say to this, except that which the Apostle said: “If God is for us, who is against us? He who spared not His own Son, but gave Him up for us all, how has He not with Him also given us all things? Who shall lay a charge against the elect? It is God Who justifies, who is he that shall condemn? It is Christ Who died, yea, Who also rose again, Who is at the right hand of God, Who also makes intercession for us.” Novatian then brings charges against those for whom Christ intercedes. Those whom Christ has redeemed unto salvation Novatian condemns to death. Those to whom Christ says: “Take My yoke upon you, and learn of Me, for I am gentle,” Novatian says, I am not gentle. On those to whom Christ says: “You shall find rest for your souls, for My yoke is pleasant and My burden is light,” Novatian lays a heavy burden and a hard yoke.
Source: On Repentance (New Advent)