Objection 1. It would seem that by Penance one sin can be pardoned without another. For it is written (Amos 4:7): "I caused it to rain upon one city, and caused it not to rain upon another city; one piece was rained upon: and the piece whereupon I rained not, withered." These words are expounded by Gregory, who says (Hom. x super Ezech.): "When a man who hates his neighbor, breaks himself of other vices, rain falls on one part of the city, leaving the other part withered, for there are some men who, when they prune some vices, become much more rooted in others." Therefore one sin can be forgiven by Penance, without another.
Objection 2. Further, Ambrose in commenting on Psalm 118, "Blessed are the undefiled in the way," after expounding verse 136 ("My eyes have sent forth springs of water"), says that "the first consolation is that God is mindful to have mercy; and the second, that He punishes, for although faith be wanting, punishment makes satisfaction and raises us up." Therefore a man can be raised up from one sin, while the sin of unbelief remains.
Objection 3. Further, when several things are not necessarily together, one can be removed without the other. Now it was stated in I-II, 73, 1 that sins are not connected together, so that one sin can be without another. Therefore also one sin can be taken away by Penance without another being taken away.
Objection 4. Further, sins are the debts, for which we pray for pardon when we say in the Lord's Prayer: "Forgive us our trespasses," etc. Now man sometimes forgives one debt without forgiving another. Therefore God also, by Penance, forgives one sin without another.
Objection 5. Further, man's sins are forgiven him through the love of God, according to Jeremiah 31:3: "I have loved thee with an everlasting love, therefore have I drawn thee, taking pity on thee." Now there is nothing to hinder God from loving a man in one respect, while being offended with him in another, even as He loves the sinner as regards his nature, while hating him for his sin. Therefore it seems possible for God, by Penance, to pardon one sin without another.
On the contrary, Augustine says in De Poenitentia [De vera et falsa Poenitentia, the authorship of which is unknown]: "There are many who repent having sinned, but not completely; for they except certain things which give them pleasure, forgetting that our Lord delivered from the devil the man who was both dumb and deaf, whereby He shows us that we are never healed unless it be from all sins."
I answer that, It is impossible for Penance to take one sin away without another. First because sin is taken away by grace removing the offense against God. Wherefore it was stated in I-II, 109, 7; I-II, 113, 2 that without grace no sin can be forgiven. Now every mortal sin is opposed to grace and excludes it. Therefore it is impossible for one sin to be pardoned without another. Secondly, because, as shown above (Article 2) mortal sin cannot be forgiven without true Penance, to which it belongs to renounce sin, by reason of its being against God, which is common to all mortal sins: and where the same reason applies, the result will be the same. Consequently a man cannot be truly penitent, if he repent of one sin and not of another. For if one particular sin were displeasing to him, because it is against the love of God above all things (which motive is necessary for true repentance), it follows that he would repent of all. Whence it follows that it is impossible for one sin to be pardoned through Penance, without another. Thirdly, because this would be contrary to the perfection of God's mercy, since His works are perfect, as stated in Deuteronomy 32:4; wherefore whomsoever He pardons, He pardons altogether. Hence Augustine says [De vera et falsa Poenitentia, the authorship of which is unknown, that "it is irreverent and heretical to expect half a pardon from Him Who is just and justice itself."
Reply to Objection 1. These words of Gregory do not refer to the forgiveness of the guilt, but to the cessation from act, because sometimes a man who has been wont to commit several kinds of sin, renounces one and not the other; which is indeed due to God's assistance, but does not reach to the pardon of the sin.
Reply to Objection 2. In this saying of Ambrose "faith" cannot denote the faith whereby we believe in Christ, because, as Augustine says on John 15:22, "If I had not come, and spoken to them, they would not have sin" (viz. unbelief): "for this is the sin which contains all others": but it stands for consciousness, because sometimes a man receives pardon for a sin of which he is not conscious, through the punishment which he bears patiently.
Reply to Objection 3. Although sins are not connected in so far as they turn towards a mutable good, yet they are connected in so far as they turn away from the immutable Good, which applies to all mortal sins in common. and it is thus that they have the character of an offense which needs to be removed by Penance.
Reply to Objection 4. Debt as regards external things, e.g. money, is not opposed to friendship through which the debt is pardoned. hence one debt can be condoned without another. On the other hand, the debt of sin is opposed to friendship, and so one sin or offense is not pardoned without another; for it would seem absurd for anyone to ask even a man to forgive him one offense and not another.
Reply to Objection 5. The love whereby God loves man's nature, does not ordain man to the good of glory from which man is excluded by any mortal sin. but the love of grace, whereby mortal sin is forgiven, ordains man to eternal life, according to Romans 6:23: "The grace of God (is) life everlasting." Hence there is no comparison.
Source: Summa Theologica (New Advent)