St. Thomas first sets forth three objections tending toward the conclusion that children of unbelievers should be baptized against the will of their parents. He then turns to elaborate his response:
On the contrary, it is written in the Decretals (Dist. xlv), quoting the council of Toledo: In regard to the Jews the holy synod commands that henceforward none of them be forced to believe; for such are not to be saved against their will, but willingly, that their righteousness may be without flaw.
I answer that, The children of unbelievers either have the use of reason or they have not. If they have, then they already begin to control their own actions, in things that are of Divine or natural law. And therefore of their own accord, and against the will of their parents, they can receive Baptism, just as they can contract marriage. Consequently such can lawfully be advised and persuaded to be baptized.
If, however, they have not yet the use of free-will, according to the natural law they are under the care of their parents as long as they cannot look after themselves. For which reason we say that even the children of the ancients were saved through the faith of their parents. Wherefore it would be contrary to natural justice if such children were baptized against their parents’ will; just as it would be if one having the use of reason were baptized against his will. Moreover under the circumstances it would be dangerous to baptize the children of unbelievers; for they would be liable to lapse into unbelief, by reason of their natural affection for their parents. Therefore it is not the custom of the Church to baptize the children of unbelievers against their will.