Objection 1. It would seem that the degrees of prophecy change as time goes on. For prophecy is directed to the knowledge of Divine things, as stated above (Article 2). Now according to Gregory (Hom. in Ezech.), "knowledge of God went on increasing as time went on." Therefore degrees of prophecy should be distinguished according to the process of time.
Objection 2. Further, prophetic revelation is conveyed by God speaking to man; while the prophets declared both in words and in writing the things revealed to them. Now it is written (1 Samuel 3:1) that before the time of Samuel "the word of the Lord was precious," i.e. rare; and yet afterwards it was delivered to many. On like manner the books of the prophets do not appear to have been written before the time of Isaias, to whom it was said (Isaiah 8:1): "Take thee a great book and write in it with a man's pen," after which many prophets wrote their prophecies. Therefore it would seem that in course of time the degree of prophecy made progress.
Objection 3. Further, our Lord said (Matthew 11:13): "The prophets and the law prophesied until John"; and afterwards the gift of prophecy was in Christ's disciples in a much more excellent manner than in the prophets of old, according to Ephesians 3:5, "In other generations" the mystery of Christ "was not known to the sons of men, as it is now revealed to His holy apostles and prophets in the Spirit." Therefore it would seem that in course of time the degree of prophecy advanced.
On the contrary, As stated above (Article 4), Moses was the greatest of the prophets, and yet he preceded the other prophets. Therefore prophecy did not advance in degree as time went on.
I answer that, As stated above (Article 2), prophecy is directed to the knowledge of Divine truth, by the contemplation of which we are not only instructed in faith, but also guided in our actions, according to Psalm 42:3, "Send forth Thy light and Thy truth: they have conducted me." Now our faith consists chiefly in two things: first, in the true knowledge of God, according to Hebrews 11:6, "He that cometh to God must believe that He is"; secondly, in the mystery of Christ's incarnation, according to John 14:1, "You believe in God, believe also in Me." Accordingly, if we speak of prophecy as directed to the Godhead as its end, it progressed according to three divisions of time, namely before the law, under the law, and under grace. For before the law, Abraham and the other patriarchs were prophetically taught things pertinent to faith in the Godhead. Hence they are called prophets, according to Psalm 104:15, "Do no evil to My prophets," which words are said especially on behalf of Abraham and Isaac. Under the Law prophetic revelation of things pertinent to faith in the Godhead was made in a yet more excellent way than hitherto, because then not only certain special persons or families but the whole people had to be instructed in these matters. Hence the Lord said to Moses (Exodus 6:2-3): "I am the Lord that appeared to Abraham, to Isaac, and to Jacob, by the name of God almighty, and My name Adonai I did not show to them"; because previously the patriarchs had been taught to believe in a general way in God, one and Almighty, while Moses was more fully instructed in the simplicity of the Divine essence, when it was said to him (Exodus 3:14): "I am Who am"; and this name is signified by Jews in the word "Adonai" on account of their veneration for that unspeakable name. Afterwards in the time of grace the mystery of the Trinity was revealed by the Son of God Himself, according to Matthew 28:19: "Going... teach ye all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost."
In each state, however, the most excellent revelation was that which was given first. Now the first revelation, before the Law, was given to Abraham, for it was at that time that men began to stray from faith in one God by turning aside to idolatry, whereas hitherto no such revelation was necessary while all persevered in the worship of one God. A less excellent revelation was made to Isaac, being founded on that which was made to Abraham. Wherefore it was said to him (Genesis 26:24): "I am the God of Abraham thy father," and in like manner to Jacob (Genesis 28:13): "I am the God of Abraham thy father, and the God of Isaac." Again in the state of the Law the first revelation which was given to Moses was more excellent, and on this revelation all the other revelations to the prophets were founded. And so, too, in the time of grace the entire faith of the Church is founded on the revelation vouchsafed to the apostles, concerning the faith in one God and three Persons, according to Matthew 16:18, "On this rock," i.e. of thy confession, "I will build My Church."
As to the faith in Christ's incarnation, it is evident that the nearer men were to Christ, whether before or after Him, the more fully, for the most part, were they instructed on this point, and after Him more fully than before, as the Apostle declares (Ephesians 3:5).
As regards the guidance of human acts, the prophetic revelation varied not according to the course of time, but according as circumstances required, because as it is written (Proverbs 29:18), "When prophecy shall fail, the people shall be scattered abroad." Wherefore at all times men were divinely instructed about what they were to do, according as it was expedient for the spiritual welfare of the elect.
Reply to Objection 1. The saying of Gregory is to be referred to the time before Christ's incarnation, as regards the knowledge of this mystery.
Reply to Objection 2. As Augustine says (De Civ. Dei xviii, 27), "just as in the early days of the Assyrian kingdom promises were made most explicitly to Abraham, so at the outset of the western Babylon," which is Rome, "and under its sway Christ was to come, in Whom were to be fulfilled the promises made through the prophetic oracles testifying in word and writing to that great event to come," the promises, namely, which were made to Abraham. "For while prophets were scarcely ever lacking to the people of Israel from the time that they began to have kings, it was exclusively for their benefit, not for that of the nations. But when those prophetic writings were being set up with greater publicity, which at some future time were to benefit the nations, it was fitting to begin when this city," Rome to wit, "was being built, which was to govern the nations."
The reason why it behooved that nation to have a number of prophets especially at the time of the kings, was that then it was not over-ridden by other nations, but had its own king; wherefore it behooved the people, as enjoying liberty, to have prophets to teach them what to do.
Reply to Objection 3. The prophets who foretold the coming of Christ could not continue further than John, who with his finger pointed to Christ actually present. Nevertheless as Jerome says on this passage, "This does not mean that there were no more prophets after John. For we read in the Acts of the Apostles that Agabus and the four maidens, daughters of Philip, prophesied." John, too, wrote a prophetic book about the end of the Church; and at all times there have not been lacking persons having the spirit of prophecy, not indeed for the declaration of any new doctrine of faith, but for the direction of human acts. Thus Augustine says (De Civ. Dei v, 26) that "the emperor Theodosius sent to John who dwelt in the Egyptian desert, and whom he knew by his ever-increasing fame to be endowed with the prophetic spirit: and from him he received a message assuring him of victory."
Source: Summa Theologica (New Advent)