For, while all things else that are born are subject to the impulse of those that beget them, the spiritual birth is dependent on the power of him who is being born. Seeing, then, that here lies the hazard, namely, that he should not miss what is for his advantage, when to every one a free choice is thus open, it were well, I think, for him who is moved towards the begetting of himself, to determine by previous reasoning what kind of father is for his advantage, and of what element it is better for him that his nature should consist.
For, as we have said, it is in the power of such a child as this to choose its parents. Since, then, there is a twofold division of existences, into created and uncreated, and since the uncreated world possesses within itself immutability and immobility, while the created is liable to change and alteration, of which will he, who with calculation and deliberation is to choose what is for his benefit, prefer to be the offspring; of that which is always found in a state of change, or of that which possesses a nature that is changeless, steadfast, and ever consistent and unvarying in goodness?
Now there have been delivered to us in the Gospel three Persons and names through whom the generation or birth of believers takes place, and he who is begotten by this Trinity is equally begotten of the Father, and of the Son, and of the Holy Ghost— for thus does the Gospel speak of the Spirit, that “that which is born of Spirit is spirit,” and it is “in Christ” that Paul begets, and the Father is the “Father of all;” here, then, I beg, let the mind of the hearer be sober in its choice, lest it make itself the offspring of some inconstant nature, when it has it in its power to make the steadfast and unalterable nature the founder of its life.
For according to the disposition of heart in one who comes to the Dispensation will that which is begotten in him exhibit its power; so that he who confesses that the Holy Trinity is uncreate enters on the steadfast unalterable life; while another, who through a mistaken conception sees only a created nature in the Trinity and then is baptized in that, has again been born into the shifting and alterable life. For that which is born is of necessity of one kindred with that which begets.
Which, then, offers the greater advantage; to enter on the unchangeable life, or to be again tossed about by the waves of this lifetime of uncertainty and change? Well, since it is evident to any one of the least understanding that what is stable is far more valuable than what is unstable, what is perfect than what is deficient, what needs not than what needs, and what has no further to advance, but ever abides in the perfection of all that is good, than what climbs by progressive toil, it is incumbent upon every one, at least upon every one who is possessed of sense, to make an absolute choice of one or other of these two conditions, either to believe that the Holy Trinity belongs to the uncreated world, and so through the spiritual birth to make It the foundation of his own life, or, if he thinks that the Son or the Holy Ghost is external to the being of the first, the true, the good, God, I mean, of the Father, not to include these Persons in the belief which he takes upon him at the moment of his new birth, lest he unconsciously make himself over to that imperfect nature which itself needs some one to make it good, and in a manner bring himself back again to something of the same nature as his own by thus removing his faith from that higher world. For whoever has bound himself to any created thing forgets that, as from the Deity, he has no longer hope of salvation. For all creation, owing to the whole equally proceeding from non-existence into being, has an intimate connection with itself; and as in the bodily organization all the limbs have a natural and mutual coherence, though some have a downward, some an upward direction, so the world of created things is, viewed as the creation, in oneness with itself, and the differences in us, as regards abundance or deficiency, in no wise disjoint it from this natural coherence with itself. For in things which equally imply the idea of a previous non-existence, though there be a difference between them in other respects, as regards this point we discover no variation of nature. If, then, man, who is himself a created being, thinks that the Spirit and the Only-begotten God are likewise created, the hope which he entertains of a change to a better state will be a vain one; for he only returns to himself. What happens then is on a par with the surmises of Nicodemus; he, when instructed by our Lord as to the necessity of being born from above, because he could not yet comprehend the meaning of the mystery, had his thoughts drawn back to his mother's womb. So that if a man does not conduct himself towards the uncreated nature, but to that which is kindred to, and equally in bondage with, himself, he is of the birth which is from below, and not of that which is from above. But the Gospel tells us that the birth of the saved is from above.
Source: The Great Catechism (New Advent)