Objection 1. It would seem that the children of Jews and of other unbelievers ought to be baptized against their parents' will. For the bond of marriage is stronger than the right of parental authority over children, since the right of parental authority can be made to cease, when a son is set at liberty; whereas the marriage bond cannot be severed by man, according to Matthew 19:6: "What... God hath joined together let no man put asunder." And yet the marriage bond is broken on account of unbelief: for the Apostle says (1 Corinthians 7:15): "If the unbeliever depart, let him depart. For a brother or sister is not under servitude in such cases": and a canon [Can. Uxor legitima, and Idololatria, qu. i] says that "if the unbelieving partner is unwilling to abide with the other, without insult to their Creator, then the other partner is not bound to cohabitation." Much more, therefore, does unbelief abrogate the right of unbelieving parents' authority over their children: and consequently their children may be baptized against their parents' will.
Objection 2. Further, one is more bound to succor a man who is in danger of everlasting death, than one who is in danger of temporal death. Now it would be a sin, if one saw a man in danger of temporal death and failed to go to his aid. Since, then, the children of Jews and other unbelievers are in danger of everlasting death, should they be left to their parents who would imbue them with their unbelief, it seems that they ought to be taken away from them and baptized, and instructed in the faith.
Objection 3. Further, the children of a bondsman are themselves bondsmen, and under the power of his master. Now the Jews are bondsmen of kings and princes: therefore their children are also. Consequently kings and princes have the power to do what they will with Jewish children. Therefore no injustice is committed if they baptize them against their parents' wishes.
Objection 4. Further, every man belongs more to God, from Whom he has his soul, than to his carnal father, from whom he has his body. Therefore it is not unjust if Jewish children be taken away from their parents, and consecrated to God in Baptism.
Objection 5. Further, Baptism avails for salvation more than preaching does, since Baptism removes forthwith the stain of sin and the debt of punishment, and opens the gate of heaven. Now if danger ensue through not preaching, it is imputed to him who omitted to preach, according to the words of Ezekiel 33:6 about the man who "sees the sword coming and sounds not the trumpet." Much more therefore, if Jewish children are lost through not being baptized are they accounted guilty of sin, who could have baptized them and did not.
On the contrary, Injustice should be done to no man. Now it would be an injustice to Jews if their children were to be baptized against their will, since they would lose the rights of parental authority over their children as soon as these were Christians. Therefore these should not be baptized against their parents' will.
I answer that, The custom of the Church has very great authority and ought to be jealously observed in all things, since the very doctrine of catholic doctors derives its authority from the Church. Hence we ought to abide by the authority of the Church rather than by that of an Augustine or a Jerome or of any doctor whatever. Now it was never the custom of the Church to baptize the children of the Jews against the will of their parents, although at times past there have been many very powerful catholic princes like Constantine and Theodosius, with whom most holy bishops have been on most friendly terms, as Sylvester with Constantine, and Ambrose with Theodosius, who would certainly not have failed to obtain this favor from them if it had been at all reasonable. It seems therefore hazardous to repeat this assertion, that the children of Jews should be baptized against their parents' wishes, in contradiction to the Church's custom observed hitherto.
There are two reasons for this custom. One is on account of the danger to the faith. For children baptized before coming to the use of reason, afterwards when they come to perfect age, might easily be persuaded by their parents to renounce what they had unknowingly embraced; and this would be detrimental to the faith.
The other reason is that it is against natural justice. For a child is by nature part of its father: thus, at first, it is not distinct from its parents as to its body, so long as it is enfolded within its mother's womb; and later on after birth, and before it has the use of its free-will, it is enfolded in the care of its parents, which is like a spiritual womb, for so long as man has not the use of reason, he differs not from an irrational animal; so that even as an ox or a horse belongs to someone who, according to the civil law, can use them when he likes, as his own instrument, so, according to the natural law, a son, before coming to the use of reason, is under his father's care. Hence it would be contrary to natural justice, if a child, before coming to the use of reason, were to be taken away from its parents' custody, or anything done to it against its parents' wish. As soon, however, as it begins to have the use of its free-will, it begins to belong to itself, and is able to look after itself, in matters concerning the Divine or the natural law, and then it should be induced, not by compulsion but by persuasion, to embrace the faith: it can then consent to the faith, and be baptized, even against its parents' wish; but not before it comes to the use of reason. Hence it is said of the children of the fathers of old that they were saved in the faith of their parents; whereby we are given to understand that it is the parents' duty to look after the salvation of their children, especially before they come to the use of reason.
Reply to Objection 1. In the marriage bond, both husband and wife have the use of the free-will, and each can assent to the faith without the other's consent. But this does not apply to a child before it comes to the use of reason: yet the comparison holds good after the child has come to the use of reason, if it is willing to be converted.
Reply to Objection 2. No one should be snatched from natural death against the order of civil law: for instance, if a man were condemned by the judge to temporal death, nobody ought to rescue him by violence: hence no one ought to break the order of the natural law, whereby a child is in the custody of its father, in order to rescue it from the danger of everlasting death.
Reply to Objection 3. Jews are bondsmen of princes by civil bondage, which does not exclude the order of natural or Divine law.
Reply to Objection 4. Man is directed to God by his reason, whereby he can know Him. Hence a child before coming to the use of reason, in the natural order of things, is directed to God by its parents' reason, under whose care it lies by nature: and it is for them to dispose of the child in all matters relating to God.
Reply to Objection 5. The peril that ensues from the omission of preaching, threatens only those who are entrusted with the duty of preaching. Hence it had already been said (Ezekiel 3:17): "I have made thee a watchman to the children [Vulgate: 'house'] of Israel." On the other hand, to provide the sacraments of salvation for the children of unbelievers is the duty of their parents. Hence it is they whom the danger threatens, if through being deprived of the sacraments their children fail to obtain salvation.
Source: Summa Theologica (New Advent)