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 Rm. 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 Rm. 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.