In How Many Ways the Creature is to Be Taken by Way of Sign. The Eucharist
Excepting, therefore, all these things as I just now said, there are some also of another kind; which, although from the same corporeal substance, are yet brought within reach of our senses in order to announce something from God, and these are properly called miracles and signs; yet is not the person of God Himself assumed in all things which are announced to us by the Lord God. When, however, that person is assumed, it is sometimes made manifest as an angel; sometimes in that form which is not an angel in his own proper being, although it is ordered and ministered by an angel. Again, when it is assumed in that form which is not an angel in his own proper being; sometimes in this case it is a body itself already existing, assumed after some kind of change, in order to make that message manifest; sometimes it is one that comes into being for the purpose, and that being accomplished, is discarded. Just as, also, when men are the messengers, sometimes they speak the words of God in their own person, as when it is premised, “The Lord said,” or, “Thus says the Lord,” or any other such phrase, but sometimes without any such prefix, they take upon themselves the very person of God, as e.g.: “I will instruct you, and teach you in the way wherein you shall go:” so, not only in word, but also in act, the signifying of the person of God is imposed upon the prophet, in order that he may bear that person in the ministering of the prophecy; just as he, for instance, bore that person who divided his garment into twelve parts, and gave ten of them to the servant of King Solomon, to the future king of Israel. Sometimes, also, a thing which was not a prophet in his own proper self, and which existed already among earthly things, was assumed in order to signify this; as Jacob, when he had seen the dream, upon waking up did with the stone, which when asleep he had under his head. Sometimes a thing is made in the same kind, for the mere purpose; so as either to continue a little while in existence, as that brazen serpent was able to do which was lifted up in the wilderness, and as written records are able to do likewise; or so as to pass away after having accomplished its ministry, as the bread made for the purpose is consumed in the receiving of the sacrament.
20. But because these things are known to men, in that they are done by men, they may well meet with reverence as being holy things, but they cannot cause wonder as being miracles. And therefore those things which are done by angels are the more wonderful to us, in that they are more difficult and more known; but they are known and easy to them as being their own actions. An angel speaks in the person of God to man, saying, “I am the God of Abraham, and the God of Isaac, and the God of Jacob;” the Scripture having said just before, “The angel of the Lord appeared to him.” And a man also speaks in the person of God, saying, “Hear, O my people, and I will testify unto you, O Israel: I am the Lord your God.” A rod was taken to serve as a sign, and was changed into a serpent by angelical power; but although that power is wanting to man, yet a stone was taken also by man for a similar sign. There is a wide difference between the deed of the angel and the deed of the man. The former is both to be wondered at and to be understood, the latter only to be understood. That which is understood from both, is perhaps one and the same; but those things from which it is understood, are different. Just as if the name of God were written both in gold and in ink; the former would be the more precious, the latter the more worthless; yet that which is signified in both is one and the same. And although the serpent that came from Moses' rod signified the same thing as Jacob's stone, yet Jacob's stone signified something better than did the serpents of the magicians. For as the anointing of the stone signified Christ in the flesh, in which He was anointed with the oil of gladness above His fellows; so the rod of Moses, turned into a serpent, signified Christ Himself made obedient unto death, even the death of the cross. Whence it is said, “And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up, that whosoever believes in Him should not perish, but have everlasting life;” just as by gazing on that serpent which was lifted up in the wilderness, they did not perish by the bites of the serpents. For “our old man is crucified with Him, that the body of sin might be destroyed.” For by the serpent death is understood, which was wrought by the serpent in paradise, the mode of speech expressing the effect by the efficient. Therefore the rod passed into the serpent, Christ into death; and the serpent again into the rod, whole Christ with His body into the resurrection; which body is the Church; and this shall be in the end of time, signified by the tail, which Moses held, in order that it might return into a rod. But the serpents of the magicians, like those who are dead in the world, unless by believing in Christ they shall have been as it were swallowed up by, and have entered into, His body, will not be able to rise again in Him. Jacob's stone, therefore, as I said, signified something better than did the serpents of the magicians; yet the deed of the magicians was much more wonderful. But these things in this way are no hindrance to the understanding of the matter; just as if the name of a man were written in gold, and that of God in ink.
Source: On the Holy Trinity (New Advent)