For when they have heard from us something to this effect— that when the mortal passes into life it follows necessarily that, as that first birth leads only to the existence of mortality, another birth should be discovered, a birth which neither begins nor ends with corruption, but one which conducts the person begotten to an immortal existence, in order that, as what is begotten of a mortal birth has necessarily a mortal subsistence, so from a birth which admits not corruption that which is born may be superior to the corruption of death; when, I say, they have heard this and the like from us, and are besides instructed as to the process—namely that it is prayer and the invocation of heavenly grace, and water, and faith, by which the mystery of regeneration is accomplished—they still remain incredulous and have an eye only for the outward and visible, as if that which is operated corporeally concurred not with the fulfilment of God's promise.
How, they ask, can prayer and the invocation of Divine power over the water be the foundation of life in those who have been thus initiated? In reply to them, unless they be of a very obstinate disposition, one single consideration suffices to bring them to an acquiescence in our doctrine. For let us in our turn ask them about that process of the carnal generation which every one can notice. How does that something which is cast for the beginnings of the formation of a living being become a Man?
In that case, most certainly, there is no method whatever that can discover for us, by any possible reasoning, even the probable truth. For what correlation is there between the definition of man and the quality observable in that something? Man, when once he is put together, is a reasoning and intellectual being, capable of thought and knowledge; but that something is to be observed only in its quality of humidity, and the mind grasps nothing in it beyond that which is seen by the sense of sight.
The reply, therefore, which we might expect to receive from those whom we questioned as to how it is credible that a man is compounded from that humid element, is the very reply which we make when questioned about the regeneration that takes place through the water. Now in that other case any one so questioned has this reply ready at hand, that that element becomes a man by a Divine power, wanting which, the element is motionless and inoperative. If, therefore, in that instance the subordinate matter does not make the man, but the Divine power changes that visible thing into a man's nature, it would be utterly unfair for them, when in the one case they testify to such power in God, in this other department to suppose that the Deity is too weak to accomplish His will.
What is there common, they ask, between water and life? What is there common, we ask them in return, between humidity and God's image? In that case there is no paradox if, God so willing, what is humid changes into the most rare creature. Equally, then, in this case we assert that there is nothing strange when the presence of a Divine influence transforms what is born with a corruptible nature into a state of incorruption.
Source: The Great Catechism (New Advent)