“But,” it is said, “this change in our body by birth is a weakness, and one born under such condition is born in weakness. Now the Deity is free from weakness. It is, therefore, a strange idea in connection with God,” they say, “when people declare that one who is essentially free from weakness thus comes into fellowship with weakness.” Now in reply to this let us adopt the same argument as before, namely that the word “weakness” is used partly in a proper, partly in an adapted sense.
Whatever, that is, affects the will and perverts it from virtue to vice is really and truly a weakness; but whatever in nature is to be seen proceeding by a chain peculiar to itself of successive stages would be more fitly called a work than a weakness. As, for instance, birth, growth, the continuance of the underlying substance through the influx and efflux of the aliments, the meeting together of the component elements of the body, and, on the other hand, the dissolution of its component parts and their passing back into the kindred elements.
Which “weakness,” then, does our Mystery assert that the Deity came in contact with? That which is properly called weakness, which is vice, or that which is the result of natural movements? Well, if our Faith affirmed that the Deity was born under forbidden circumstances, then it would be our duty to shun a statement which gave this profane and unsound description of the Divine Being. But if it asserts that God laid hold on this nature of ours, the production of which in the first instance and the subsistence afterwards had its origin in Him, in what way does this our preaching fail in the reverence that befits Him?
Amongst our notions of God no disposition tending to weakness goes along with our belief in Him. We do not say that a physician is in weakness when he is employed in healing one who is so. For though he touches the infirmity he is himself unaffected by it. If birth is not regarded in itself as a weakness, no one can call life such. But the feeling of sensual pleasure does go before the human birth, and as to the impulse to vice in all living men, this is a disease of our nature.
But then the Gospel mystery asserts that He Who took our nature was pure from both these feelings. If, then, His birth had no connection with sensual pleasure, and His life none with vice, what “weakness” is there left which the mystery of our religion asserts that God participated in? But should any one call the separation of body and soul a weakness, far more justly might he term the meeting together of these two elements such. For if the severance of things that have been connected is a weakness, then is the union of things that are asunder a weakness also.
For there is a feeling of movement in the uniting of things sundered as well as in the separation of what has been welded into one. The same term, then, by which the final movement is called, it is proper to apply to the one that initiated it. If the first movement, which we call birth, is not a weakness, it follows that neither the second, which we call death, and by which the severance of the union of the soul and body is effected, is a weakness. Our position is, that God was born subject to both movements of our nature; first, that by which the soul hastens to join the body, and then again that by which the body is separated from the soul; and that when the concrete humanity was formed by the mixture of these two, I mean the sentient and the intelligent element, through that ineffable and inexpressible conjunction, this result in the Incarnation followed, that after the soul and body had been once united the union continued for ever.
For when our nature, following its own proper course, had even in Him been advanced to the separation of soul and body, He knitted together again the disunited elements, cementing them, as it were, together with the cement of His Divine power, and recombining what has been severed in a union never to be broken. And this is the Resurrection, namely the return, after they have been dissolved, of those elements that had been before linked together, into an indissoluble union through a mutual incorporation; in order that thus the primal grace which invested humanity might be recalled, and we restored to the everlasting life, when the vice that has been mixed up with our kind has evaporated through our dissolution, as happens to any liquid when the vessel that contained it is broken, and it is spilt and disappears, there being nothing to contain it.
For as the principle of death took its rise in one person and passed on in succession through the whole of human kind, in like manner the principle of the Resurrection-life extends from one person to the whole of humanity. For He Who reunited to His own proper body the soul that had been assumed by Himself, by virtue of that power which had mingled with both of these component elements at their first framing, then, upon a more general scale as it were, conjoined the intellectual to the sentient nature, the new principle freely progressing to the extremities by natural consequence.
For when, in that concrete humanity which He had taken to Himself, the soul after the dissolution returned to the body, then this uniting of the several portions passes, as by a new principle, in equal force upon the whole human race. This, then, is the mystery of God's plan with regard to His death and His resurrection from the dead; namely, instead of preventing the dissolution of His body by death and the necessary results of nature, to bring both back to each other in the resurrection; so that He might become in Himself the meeting-ground both of life and death, having re-established in Himself that nature which death had divided, and being Himself the originating principle of the uniting those separated portions.
Source: The Great Catechism (New Advent)