But since the human being is a twofold creature, compounded of soul and body, it is necessary that the saved should lay hold of the Author of the new life through both their component parts. Accordingly, the soul being fused into Him through faith derives from that the means and occasion of salvation; for the act of union with the life implies a fellowship with the life. But the body comes into fellowship and blending with the Author of our salvation in another way. For as they who owing to some act of treachery have taken poison, allay its deadly influence by means of some other drug (for it is necessary that the antidote should enter the human vitals in the same way as the deadly poison, in order to secure, through them, that the effect of the remedy may be distributed through the entire system), in like manner we, who have tasted the solvent of our nature, necessarily need something that may combine what has been so dissolved, so that such an antidote entering within us may, by its own counter-influence, undo the mischief introduced into the body by the poison.
What, then, is this remedy to be? Nothing else than that very Body which has been shown to be superior to death, and has been the First-fruits of our life. For, in the manner that, as the Apostle says, a little leaven assimilates to itself the whole lump, so in like manner that body to which immortality has been given it by God, when it is in ours, translates and transmutes the whole into itself. For as by the admixture of a poisonous liquid with a wholesome one the whole drought is deprived of its deadly effect, so too the immortal Body, by being within that which receives it, changes the whole to its own nature.
Yet in no other way can anything enter within the body but by being transfused through the vitals by eating and drinking. It is, therefore, incumbent on the body to admit this life-producing power in the one way that its constitution makes possible. And since that Body only which was the receptacle of the Deity received this grace of immortality, and since it has been shown that in no other way was it possible for our body to become immortal, but by participating in incorruption through its fellowship with that immortal Body, it will be necessary to consider how it was possible that that one Body, being for ever portioned to so many myriads of the faithful throughout the whole world, enters through that portion, whole into each individual, and yet remains whole in itself.
In order, therefore, that our faith, with eyes fixed on logical probability, may harbour no doubt on the subject before us, it is fitting to make a slight digression in our argument, to consider the physiology of the body. Who is there that does not know that our bodily frame, taken by itself, possesses no life in its own proper subsistence, but that it is by the influx of a force or power from without that it holds itself together and continues in existence, and by a ceaseless motion that it draws to itself what it wants, and repels what is superfluous?
When a leathern bottle is full of some liquid, and then the contents leak out at the bottom, it would not retain the contour of its full bulk unless there entered in at the top something else to fill up the vacuum; and thus a person, seeing the circumference of this bottle swollen to its full size, would know that this circumference did not really belong to the object which he sees, but that what was being poured in, by being in it, gave shape and roundness to the bulk. In the same way the mere framework of our body possesses nothing belonging to itself that is cognizable by us, to hold it together, but remains in existence owing to a force that is introduced into it.
Now this power or force both is, and is called, nourishment. But it is not the same in all bodies that require aliment, but to each of them has been assigned a food adapted to its condition by Him who governs Nature. Some animals feed on roots which they dig up. Of others grass is the food, of others different kinds of flesh, but for man above all things bread; and, in order to continue and preserve the moisture of his body, drink, not simply water, but water frequently sweetened with wine, to join forces with our internal heat.
He, therefore, who thinks of these things, thinks by implication of the particular bulk of our body. For those things by being within me became my blood and flesh, the corresponding nutriment by its power of adaptation being changed into the form of my body. With these distinctions we must return to the consideration of the question before us. The question was, how can that one Body of Christ vivify the whole of mankind, all, that is, in whomsoever there is Faith, and yet, though divided among all, be itself not diminished?
Perhaps, then, we are now not far from the probable explanation. If the subsistence of every body depends on nourishment, and this is eating and drinking, and in the case of our eating there is bread and in the case of our drinking water sweetened with wine, and if, as was explained at the beginning, the Word of God, Who is both God and the Word, coalesced with man's nature, and when He came in a body such as ours did not innovate on man's physical constitution so as to make it other than it was, but secured continuance for His own body by the customary and proper means, and controlled its subsistence by meat and drink, the former of which was bread—just, then, as in the case of ourselves, as has been repeatedly said already, if a person sees bread he also, in a kind of way, looks on a human body, for by the bread being within it the bread becomes it, so also, in that other case, the body into which God entered, by partaking of the nourishment of bread, was, in a certain measure, the same with it; that nourishment, as we have said, changing itself into the nature of the body.
For that which is peculiar to all flesh is acknowledged also in the case of that flesh, namely, that that Body too was maintained by bread; which Body also by the indwelling of God the Word was transmuted to the dignity of Godhead. Rightly, then, do we believe that now also the bread which is consecrated by the Word of God is changed into the Body of God the Word. For that Body was once, by implication, bread, but has been consecrated by the inhabitation of the Word that tabernacled in the flesh.
Therefore, from the same cause as that by which the bread that was transformed in that Body was changed to a Divine potency, a similar result takes place now. For as in that case, too, the grace of the Word used to make holy the Body, the substance of which came of the bread, and in a manner was itself bread, so also in this case the bread, as says the Apostle, “is sanctified by the Word of God and prayer”; not that it advances by the process of eating to the stage of passing into the body of the Word, but it is at once changed into the body by means of the Word, as the Word itself said, “This is My Body.”
Seeing, too, that all flesh is nourished by what is moist (for without this combination our earthly part would not continue to live), just as we support by food which is firm and solid the solid part of our body, in like manner we supplement the moist part from the kindred element; and this, when within us, by its faculty of being transmitted, is changed to blood, and especially if through the wine it receives the faculty of being transmuted into heat. Since, then, that God-containing flesh partook for its substance and support of this particular nourishment also, and since the God who was manifested infused Himself into perishable humanity for this purpose, viz. that by this communion with Deity mankind might at the same time be deified, for this end it is that, by dispensation of His grace, He disseminates Himself in every believer through that flesh, whose substance comes from bread and wine, blending Himself with the bodies of believers, to secure that, by this union with the immortal, man, too, may be a sharer in incorruption.
He gives these gifts by virtue of the benediction through which He transelements the natural quality of these visible things to that immortal thing.
Source: The Great Catechism (New Advent)