Concerning the holyand immaculate Mysteries of the Lord
God Who is good and altogether good and more than good, Who is goodness throughout, by reason of the exceeding riches of His goodness did not suffer Himself, that is His nature, only to be good, with no other to participate therein, but because of this He made first the spiritual and heavenly powers: next the visible and sensible universe: next man with his spiritual and sentient nature. All things, therefore, which he made, share in His goodness in respect of their existence. For He Himself is existence to all, since all things that are, are in Him, not only because it was He that brought them out of nothing into being, but because His energy preserves and maintains all that He made: and in special the living creatures. For both in that they exist and in that they enjoy life they share in His goodness. But in truth those of them that have reason have a still greater share in that, both because of what has been already said and also because of the very reason which they possess. For they are somehow more dearly akin to Him, even though He is incomparably higher than they.
Man, however, being endowed with reason and free will, received the power of continuous union with God through his own choice, if indeed he should abide in goodness, that is in obedience to his Maker. Since, however, he transgressed the command of his Creator and became liable to death and corruption, the Creator and Maker of our race, because of His bowels of compassion, took on our likeness, becoming man in all things but without sin, and was united to our nature. For since He bestowed on us His own image and His own spirit and we did not keep them safe, He took Himself a share in our poor and weak nature, in order that He might cleanse us and make us incorruptible, and establish us once more as partakers of His divinity.
For it was fitting that not only the first-fruits of our nature should partake in the higher good but every man who wished it, and that a second birth should take place and that the nourishment should be new and suitable to the birth and thus the measure of perfection be attained. Through His birth, that is, His incarnation, and baptism and passion and resurrection, He delivered our nature from the sin of our first parent and death and corruption, and became the first-fruits of the resurrection, and made Himself the way and image and pattern, in order that we, too, following in His footsteps, may become by adoption what He is Himself by nature, sons and heirs of God and joint heirs with Him. He gave us therefore, as I said, a second birth in order that, just as we who are born of Adam are in his image and are the heirs of the curse and corruption, so also being born of Him we may be in His likeness and heirs of His incorruption and blessing and glory.
Now seeing that this Adam is spiritual, it was meet that both the birth and likewise the food should be spiritual too, but since we are of a double and compound nature, it is meet that both the birth should be double and likewise the food compound. We were therefore given a birth by water and Spirit: I mean, by the holy baptism: and the food is the very bread of life, our Lord Jesus Christ, Who came down from heaven. For when He was about to take on Himself a voluntary death for our sakes, on the night on which He gave Himself up, He laid a new covenant on His holy disciples and apostles, and through them on all who believe in Him. In the upper chamber, then, of holy and illustrious Sion, after He had eaten the ancient Passover with His disciples and had fulfilled the ancient covenant, He washed His disciples' feet in token of the holy baptism. Then having broken bread He gave it to them saying, Take, eat, this is My body broken for you for the remission of sins. Likewise also He took the cup of wine and water and gave it to them saying, Drink ye all of it: for this is My blood, the blood of the New Testament which is shed for you for the remission of sins. This do ye in remembrance of Me. For as often as you eat this bread and drink this cup, you do show the death of the Son of man and confess His resurrection until He come.
If then the Word of God is quick and energising, and the Lord did all that He willed; if He said, Let there be light and there was light, let there be a firmament and there was a firmament; if the heavens were established by the Word of the Lord and all the host of them by the breath of His mouth; if the heaven and the earth, water and fire and air and the whole glory of these, and, in truth, this most noble creature, man, were perfected by the Word of the Lord; if God the Word of His own will became man and the pure and undefiled blood of the holy and ever-virginal One made His flesh without the aid of seed, can He not then make the bread His body and the wine and water His blood? He said in the beginning, Let the earth bring forth grass, and even until this present day, when the rain comes it brings forth its proper fruits, urged on and strengthened by the divine command. God said, This is My body, and This is My blood, and this do ye in remembrance of Me. And so it is at His omnipotent command until He come: for it was in this sense that He said until He come: and the overshadowing power of the Holy Spirit becomes through the invocation the rain to this new tillage. For just as God made all that He made by the energy of the Holy Spirit, so also now the energy of the Spirit performs those things that are supernatural and which it is not possible to comprehend unless by faith alone. How shall this be, said the holy Virgin, seeing I know not a man? And the archangel Gabriel answered her: The Holy Spirit shall come upon you, and the power of the Highest shall overshadow you. And now you ask, how the bread became Christ's body and the wine and water Christ's blood. And I say unto you, “The Holy Spirit is present and does those things which surpass reason and thought.”
Further, bread and wine are employed: for God knows man's infirmity: for in general man turns away discontentedly from what is not well-worn by custom: and so with His usual indulgence He performs His supernatural works through familiar objects: and just as, in the case of baptism, since it is man's custom to wash himself with water and anoint himself with oil, He connected the grace of the Spirit with the oil and the water and made it the water of regeneration, in like manner since it is man's custom to eat and to drink water and wine, He connected His divinity with these and made them His body and blood in order that we may rise to what is supernatural through what is familiar and natural.
The body which is born of the holy Virgin is in truth body united with divinity, not that the body which was received up into the heavens descends, but that the bread itself and the wine are changed into God's body and blood. But if you enquire how this happens, it is enough for you to learn that it was through the Holy Spirit, just as the Lord took on Himself flesh that subsisted in Him and was born of the holy Mother of God through the Spirit. And we know nothing further save that the Word of God is true and energises and is omnipotent, but the manner of this cannot be searched out. But one can put it well thus, that just as in nature the bread by the eating and the wine and the water by the drinking are changed into the body and blood of the eater and drinker, and do not become a different body from the former one, so the bread of the table and the wine and water are supernaturally changed by the invocation and presence of the Holy Spirit into the body and blood of Christ, and are not two but one and the same.
Source: Exposition of the Faith (New Advent)