Objection 1. It would seem that man is not restored by Penance to his former dignity: because a gloss on Amos 5:2, "The virgin of Israel is cast down," observes: "It is not said that she cannot rise up, but that the virgin of Israel shall not rise; because the sheep that has once strayed, although the shepherd bring it back on his shoulder, has not the same glory as if it had never strayed." Therefore man does not, through Penance, recover his former dignity.
Objection 2. Further, Jerome says: "Whoever fail to preserve the dignity of the sacred order, must be content with saving their souls; for it is a difficult thing to return to their former degree." Again, Pope Innocent I says (Ep. vi ad Agapit.) that "the canons framed at the council of Nicaea exclude penitents from even the lowest orders of clerics." Therefore man does not, through Penance, recover his former dignity.
Objection 3. Further, before sinning a man can advance to a higher sacred order. But this is not permitted to a penitent after his sin, for it is written (Ezekiel 44:10-13): "The Levites that went away... from Me... shall never [Vulgate: 'not'] come near to Me, to do the office of priest": and as laid down in the Decretals (Dist. 1, ch. 52), and taken from the council of Lerida: "If those who serve at the Holy Altar fall suddenly into some deplorable weakness of the flesh, and by God's mercy do proper penance, let them return to their duties, yet so as not to receive further promotion." Therefore Penance does not restore man to his former dignity.
On the contrary, As we read in the same Distinction, Gregory writing to Secundinus (Regist. vii) says: "We consider that when a man has made proper satisfaction, he may return to his honorable position": and moreover we read in the acts of the council of Agde: "Contumacious clerics, so far as their position allows, should be corrected by their bishops. so that when Penance has reformed them, they may recover their degree and dignity."
I answer that, By sin, man loses a twofold dignity, one in respect of God, the other in respect of the Church. In respect of God he again loses a twofold dignity. one is his principal dignity, whereby he was counted among the children of God, and this he recovers by Penance, which is signified (Luke 15) in the prodigal son, for when he repented, his father commanded that the first garment should be restored to him, together with a ring and shoes. The other is his secondary dignity, viz. innocence, of which, as we read in the same chapter, the elder son boasted saying (Luke 15:29): "Behold, for so many years do I serve thee, and I have never transgressed thy commandments": and this dignity the penitent cannot recover. Nevertheless he recovers something greater sometimes; because as Gregory says (Hom. de centum Ovibus, 34 in Evang.), "those who acknowledge themselves to have strayed away from God, make up for their past losses, by subsequent gains: so that there is more joy in heaven on their account, even as in battle, the commanding officer thinks more of the soldier who, after running away, returns and bravely attacks the foe, than of one who has never turned his back, but has done nothing brave."
By sin man loses his ecclesiastical dignity, because thereby he becomes unworthy of those things which appertain to the exercise of the ecclesiastical dignity. This he is debarred from recovering: first, because he fails to repent; wherefore Isidore wrote to the bishop Masso, and as we read in the Distinction quoted above (Objection 3): "The canons order those to be restored to their former degree, who by repentance have made satisfaction for their sins, or have made worthy confession of them. On the other hand, those who do not mend their corrupt and wicked ways are neither allowed to exercise their order, nor received to the grace of communion."
Secondly, because he does penance negligently, wherefore it is written in the same Distinction (Objection 3): "We can be sure that those who show no signs of humble compunction, or of earnest prayer, who avoid fasting or study, would exercise their former duties with great negligence if they were restored to them."
Thirdly, if he has committed a sin to which an irregularity is attached; wherefore it is said in the same Distinction (Objection 3), quoting the council of Pope Martin [Martin, bishop of Braga]: "If a man marry a widow or the relict of another, he must not be admitted to the ranks of the clergy: and if he has succeeded in creeping in, he must be turned out. In like manner, if anyone after Baptism be guilty of homicide, whether by deed, or by command, or by counsel, or in self-defense." But this is in consequence not of sin, but of irregularity.
Fourthly, on account of scandal, wherefore it is said in the same Distinction (Objection 3): "Those who have been publicly convicted or caught in the act of perjury, robbery, fornication, and of such like crimes, according to the prescription of the sacred canons must be deprived of the exercise of their respective orders, because it is a scandal to God's people that such persons should be placed over them. But those who commit such sins occultly and confess them secretly to a priest, may be retained in the exercise of their respective orders, with the assurance of God's merciful forgiveness, provided they be careful to expiate their sins by fasts and alms, vigils and holy deeds." The same is expressed (Extra, De Qual. Ordinand.): "If the aforesaid crimes are not proved by a judicial process, or in some other way made notorious, those who are guilty of them must not be hindered, after they have done penance, from exercising the orders they have received, or from receiving further orders, except in cases of homicide."
Reply to Objection 1. The same is to be said of the recovery of virginity as of the recovery of innocence which belongs to man's secondary dignity in the sight of God.
Reply to Objection 2. In these words Jerome does not say that it is impossible, but that it is difficult, for man to recover his former dignity after having sinned, because this is allowed to none but those who repent perfectly, as stated above. To those canonical statutes, which seem to forbid this, Augustine replies in his letter to Boniface (Ep. clxxxv): "If the law of the Church forbids anyone, after doing penance for a crime, to become a cleric, or to return to his clerical duties, or to retain them the intention was not to deprive him of the hope of pardon, but to preserve the rigor of discipline; else we should have to deny the keys given to the Church, of which it was said: 'Whatsoever you shall loose on earth shall be loosed in heaven.'" And further on he adds: "For holy David did penance for his deadly crimes, and yet he retained his dignity; and Blessed Peter by shedding most bitter tears did indeed repent him of having denied his Lord, and yet he remained an apostle. Nevertheless we must not deem the care of later teachers excessive, who without endangering a man's salvation, exacted more from his humility, having, in my opinion, found by experience, that some assumed a pretended repentance through hankering after honors and power."
Reply to Objection 3. This statute is to be understood as applying to those who do public penance, for these cannot be promoted to a higher order. For Peter, after his denial, was made shepherd of Christ's sheep, as appears from John 21:21, where Chrysostom comments as follows: "After his denial and repentance Peter gives proof of greater confidence in Christ: for whereas, at the supper, he durst not ask Him, but deputed John to ask in his stead, afterwards he was placed at the head of his brethren, and not only did not depute another to ask for him, what concerned him, but henceforth asks the Master instead of John."
Source: Summa Theologica (New Advent)