But they ask for proof of this presence of the Deity when invoked for the sanctification of the baptismal process. Let the person who requires this evidence recall to mind the result of our inquiries further back. The reasoning by which we established that the power which was manifested to us through the flesh was really a Divine power, is the defence of that which we now say. For when it has been shown that He Who was manifested in the flesh, and then exhibited His nature by the miracles which He wrought, was God, it is also at the same time shown that He is present in that process, as often as He is invoked.
For, as of everything that exists there is some peculiarity which indicates its nature, so truth is the distinctive peculiarity of the Divine nature. Well, then, He has promised that He will always be present with those that call upon Him, that He is in the midst of those that believe, that He remains among them collectively and has special intercourse with each one. We can no longer, then, need any other proof of the presence of the Deity in the things that are done in Baptism, believing as we do that He is God by reason of the miracles which He wrought, and knowing as we do that it is the peculiarity of the Godhead to be free from any touch of falsehood, and confidently holding as we do that the thing promised was involved in the truthfulness of its announcement.
The invocation by prayer, then, which precedes this Divine Dispensation constitutes an abundance of proof that what is effected is done by God. For if in the case of that other kind of man-formation the impulses of the parents, even though they do not invoke the Deity, yet by the power of God, as we have before said, mould the embryo, and if this power is withheld their eagerness is ineffectual and useless, how much more will the object be accomplished in that spiritual mode of generation, where both God has promised that He will be present in the process and, as we have believed, has put power from Himself into the work, and, besides, our own will is bent upon that object; supposing, that is, that the aid which comes through prayer has at the same time been duly called in?
For as they who pray God that the sun may shine on them in no way blunt the promptitude of that which is actually going to take place, yet no one will say that the zeal of those who thus pray is useless on the ground that they pray God for what must happen, in the same way they who, resting on the truthfulness of His promise, are firmly persuaded that His grace is surely present in those who are regenerate in this mystical Dispensation, either themselves make an actual addition to that grace, or at all events do not cause the existing grace to miscarry.
For that the grace is there is a matter of faith, on account of Him Who has promised to give it being Divine; while the testimony as to His Divinity comes through the Miracles. Thus, then, that the Deity is present in all the baptismal process admits of no question.
Source: The Great Catechism (New Advent)