Objection 1. It would seem that the Church ought in all cases to receive those who return from heresy. For it is written (Jeremiah 3:1) in the person of the Lord: "Thou hast prostituted thyself to many lovers; nevertheless return to Me saith the Lord." Now the sentence of the Church is God's sentence, according to Deuteronomy 1:17: "You shall hear the little as well as the great: neither shall you respect any man's person, because it is the judgment of God." Therefore even those who are guilty of the prostitution of unbelief which is spiritual prostitution, should be received all the same.
Objection 2. Further, Our Lord commanded Peter (Matthew 18:22) to forgive his offending brother "not" only "till seven times, but till seventy times seven times," which Jerome expounds as meaning that "a man should be forgiven, as often as he has sinned." Therefore he ought to be received by the Church as often as he has sinned by falling back into heresy.
Objection 3. Further, heresy is a kind of unbelief. Now other unbelievers who wish to be converted are received by the Church. Therefore heretics also should be received.
On the contrary, The Decretal Ad abolendam (De Haereticis, cap. ix) says that "those who are found to have relapsed into the error which they had already abjured, must be left to the secular tribunal." Therefore they should not be received by the Church.
I answer that, In obedience to Our Lord's institution, the Church extends her charity to all, not only to friends, but also to foes who persecute her, according to Matthew 5:44: "Love your enemies; do good to them that hate you." Now it is part of charity that we should both wish and work our neighbor's good. Again, good is twofold: one is spiritual, namely the health of the soul, which good is chiefly the object of charity, since it is this chiefly that we should wish for one another. Consequently, from this point of view, heretics who return after falling no matter how often, are admitted by the Church to Penance whereby the way of salvation is opened to them.
The other good is that which charity considers secondarily, viz. temporal good, such as life of the body, worldly possessions, good repute, ecclesiastical or secular dignity, for we are not bound by charity to wish others this good, except in relation to the eternal salvation of them and of others. Hence if the presence of one of these goods in one individual might be an obstacle to eternal salvation in many, we are not bound out of charity to wish such a good to that person, rather should we desire him to be without it, both because eternal salvation takes precedence of temporal good, and because the good of the many is to be preferred to the good of one. Now if heretics were always received on their return, in order to save their lives and other temporal goods, this might be prejudicial to the salvation of others, both because they would infect others if they relapsed again, and because, if they escaped without punishment, others would feel more assured in lapsing into heresy. For it is written (Ecclesiastes 8:11): "For because sentence is not speedily pronounced against the evil, the children of men commit evils without any fear."
For this reason the Church not only admits to Penance those who return from heresy for the first time, but also safeguards their lives, and sometimes by dispensation, restores them to the ecclesiastical dignities which they may have had before, should their conversion appear to be sincere: we read of this as having frequently been done for the good of peace. But when they fall again, after having been received, this seems to prove them to be inconstant in faith, wherefore when they return again, they are admitted to Penance, but are not delivered from the pain of death.
Reply to Objection 1. In God's tribunal, those who return are always received, because God is a searcher of hearts, and knows those who return in sincerity. But the Church cannot imitate God in this, for she presumes that those who relapse after being once received, are not sincere in their return; hence she does not debar them from the way of salvation, but neither does she protect them from the sentence of death.
Reply to Objection 2. Our Lord was speaking to Peter of sins committed against oneself, for one should always forgive such offenses and spare our brother when he repents. These words are not to be applied to sins committed against one's neighbor or against God, for it is not left to our discretion to forgive such offenses, as Jerome says on Matthew 18:15, "If thy brother shall offend against thee." Yet even in this matter the law prescribes limits according as God's honor or our neighbor's good demands.
Reply to Objection 3. When other unbelievers, who have never received the faith are converted, they do not as yet show signs of inconstancy in faith, as relapsed heretics do; hence the comparison fails.
Source: Summa Theologica (New Advent)