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 Ps. 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 Heb. 11:6, "He that cometh to God must believe that He is"; secondly, in the mystery of Christ's incarnation, according to Jn. 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 Ps. 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 (Ex. 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 (Ex. 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 Mt. 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 (Gn. 26:24): "I am the God of Abraham thy father," and in like manner to Jacob (Gn. 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 Mt. 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 (Eph. 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 (Prov. 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.