Objection 1. It would seem that sins once forgiven return through a subsequent sin. For Augustine says (De Bapt. contra Donat. i, 12): "Our Lord teaches most explicitly in the Gospel that sins which have been forgiven return, when fraternal charity ceases, in the example of the servant from whom his master exacted the payment of the debt already forgiven, because he had refused to forgive the debt of his fellow-servant." Now fraternal charity is destroyed through each mortal sin. Therefore sins already taken away through Penance, return through each subsequent mortal sin.
Objection 2. Further, on Luke 11:24, "I will return into my house, whence I came out," Bede says: "This verse should make us tremble, we should not endeavor to explain it away lest through carelessness we give place to the sin which we thought to have been taken away, and become its slave once more." Now this would not be so unless it returned. Therefore a sin returns after once being taken away by Penance.
Objection 3. Further, the Lord said (Ezekiel 18:24): "If the just man turn himself away from his justice, and do iniquity... all his justices which he hath done, shall not be remembered." Now among the other "justices" which he had done, is also his previous penance, since it was said above (Question 85, Article 3) that penance is a part of justice. Therefore when one who has done penance, sins, his previous penance, whereby he received forgiveness of his sins, is not imputed to him. Therefore his sins return.
Objection 4. Further, past sins are covered by grace, as the Apostle declares (Romans 4:7) where he quotes Psalm 31:1: "Blessed are they whose iniquities are forgiven, and whose sins are covered." But a subsequent mortal sin takes away grace. Therefore the sins committed previously, become uncovered: and so, seemingly, they return.
On the contrary, The Apostle says (Romans 11:29): "The gifts and the calling of God are without repentance." Now the penitent's sins are taken away by a gift of God. Therefore the sins which have been taken away do not return through a subsequent sin, as though God repented His gift of forgiveness.
Moreover, Augustine says (Lib. Resp. Prosperi i [Cf. Prosper, Responsiones ad Capitula Gallorum ii): "When he that turns away from Christ, comes to the end of this life a stranger to grace, whither does he go, except to perdition? Yet he does not fall back into that which had been forgiven, nor will he be condemned for original sin."
I answer that, As stated above (Question 86, Article 4), mortal sin contains two things, aversion from God and adherence to a created good. Now, in mortal sin, whatever attaches to the aversion, is, considered in itself, common to all mortal sins, since man turns away from God by every mortal sin, so that, in consequence, the stain resulting from the privation of grace, and the debt of everlasting punishment are common to all mortal sins. This is what is meant by what is written (James 2:10): "Whosoever... shall offend in one point, is become guilty of all." On the other hand, as regards their adherence they are different from, and sometimes contrary to one another. Hence it is evident, that on the part of the adherence, a subsequent mortal sin does not cause the return of mortal sins previously dispelled, else it would follow that by a sin of wastefulness a man would be brought back to the habit or disposition of avarice previously dispelled, so that one contrary would be the cause of another, which is impossible. But if in mortal sins we consider that which attaches to the aversion absolutely, then a subsequent mortal sin [causes the return of that which was comprised in the mortal sins before they were pardoned, in so far as the subsequent mortal sin] [The words in brackets are omitted in the Leonine edition.] deprives man of grace, and makes him deserving of everlasting punishment, just as he was before. Nevertheless, since the aversion of mortal sin is [in a way, caused by the adherence, those things which attach to the aversion are*] diversified somewhat in relation to various adherences, as it were to various causes, so that there will be a different aversion, a different stain, a different debt of punishment, according to the different acts of mortal sin from which they arise; hence the question is moved whether the stain and the debt of eternal punishment, as caused by acts of sins previously pardoned, return through a subsequent mortal sin.
Accordingly some have maintained that they return simply even in this way. But this is impossible, because what God has done cannot be undone by the work of man. Now the pardon of the previous sins was a work of Divine mercy, so that it cannot be undone by man's subsequent sin, according to Romans 3:3: "Shall their unbelief make the faith of God without effect?"
Wherefore others who maintained the possibility of sins returning, said that God pardons the sins of a penitent who will afterwards sin again, not according to His foreknowledge, but only according to His present justice: since He foresees that He will punish such a man eternally for his sins, and yet, by His grace, He makes him righteous for the present. But this cannot stand: because if a cause be placed absolutely, its effect is placed absolutely; so that if the remission of sins were effected by grace and the sacraments of grace, not absolutely but under some condition dependent on some future event, it would follow that grace and the sacraments of grace are not the sufficient causes of the remission of sins, which is erroneous, as being derogatory to God's grace.
Consequently it is in no way possible for the stain of past sins and the debt of punishment incurred thereby, to return, as caused by those acts. Yet it may happen that a subsequent sinful act virtually contains the debt of punishment due to the previous sin, in so far as when a man sins a second time, for this very reason he seems to sin more grievously than before, as stated in Romans 2:5: "According to thy hardness and impenitent heart, thou treasurest up to thyself wrath against the day of wrath," from the mere fact, namely, that God's goodness, which waits for us to repent, is despised. And so much the more is God's goodness despised, if the first sin is committed a second time after having been forgiven, as it is a greater favor for the sin to be forgiven than for the sinner to be endured.
Accordingly the sin which follows repentance brings back, in a sense, the debt of punishment due to the sins previously forgiven, not as caused by those sins already forgiven but as caused by this last sin being committed, on account of its being aggravated in view of those previous sins. This means that those sins return, not simply, but in a restricted sense, viz., in so far as they are virtually contained in the subsequent sin.
Reply to Objection 1. This saying of Augustine seems to refer to the return of sins as to the debt of eternal punishment considered in itself, namely, that he who sins after doing penance incurs a debt of eternal punishment, just as before, but not altogether for the same "reason." Wherefore Augustine, after saying (Lib. Resp. Prosperi i [Cf. Prosper, Responsiones ad Capitula Gallorum ii) that "he does not fall back into that which was forgiven, nor will he be condemned for original sin," adds: "Nevertheless, for these last sins he will be condemned to the same death, which he deserved to suffer for the former," because he incurs the punishment of eternal death which he deserved for his previous sins.
Reply to Objection 2. By these words Bede means that the guilt already forgiven enslaves man, not by the return of his former debt of punishment, but by the repetition of his act.
Reply to Objection 3. The effect of a subsequent sin is that the former "justices" are not remembered, in so far as they were deserving of eternal life, but not in so far as they were a hindrance to sin. Consequently if a man sins mortally after making restitution, he does not become guilty as though he had not paid back what he owed; and much less is penance previously done forgotten as to the pardon of the guilt, since this is the work of God rather than of man.
Reply to Objection 4. Grace removes the stain and the debt of eternal punishment simply; but it covers the past sinful acts, lest, on their account, God deprive man of grace, and judge him deserving of eternal punishment; and what grace has once done, endures for ever.
Source: Summa Theologica (New Advent)