Further, that the Same Rule of Truth Teaches Us to Believe, After the Father, Also in the Son of God, Jesus Christ Our Lord God, Being the Same that Was Promised in the Old Testament, and Manifested in the New.
The same rule of truth teaches us to believe, after the Father, also on the Son of God, Christ Jesus, the Lord our God, but the Son of God— of that God who is both one and alone, to wit the Founder of all things, as already has been expressed above. For this Jesus Christ, I will once more say, the Son of this God, we read of as having been promised in the Old Testament, and we observe to be manifested in the New, fulfilling the shadows and figures of all the sacraments, with the presence of the truth embodied. For as well the ancient prophecies as the Gospels testify Him to be the son of Abraham and the son of David. Genesis itself anticipates Him, when it says: “To you will I give it, and to your seed.” He is spoken of when it shows how a man wrestled with Jacob; He too, when it says: “There shall not fail a prince from Judah, nor a leader from between his thighs, until He shall come to whom it has been promised; and He shall be the expectation of the nations.” He is spoken of by Moses when he says: “Provide another whom you may send.” He is again spoken of by the same, when he testifies, saying: “A Prophet will God raise up to you from your brethren; listen to Him as if to me.” It is He, too, that he speaks of when he says: “You shall see your life hanging in doubt night and day, and you shall not believe Him.” Him, too, Isaiah alludes to: “There shall go forth a rod from the root of Jesse, and a flower shall grow up from his root.” The same also when he says: “Behold, a virgin shall conceive, and bear a son.” Him he refers to when he enumerates the healings that were to proceed from Him, saying: “Then shall the eyes of the blind be opened, and the ears of the deaf shall hear: then shall the lame man leap as an hart, and the tongue of the dumb shall be eloquent.” Him also, when he sets forth the virtue of patience, saying: “His voice shall not be heard in the streets; a bruised reed shall He not destroy, and the smoking flax shall He not quench.” Him, too, when he described His Gospel: “And I will ordain for you an everlasting covenant, even the sure mercies of David.” Him, too, when he foretells that the nations should believe in Him: “Behold, I have given Him for a Chief and a Commander to the nations. Nations that knew not You shall call upon You, and peoples that knew You not shall flee unto You.” It is the same that he refers to when, concerning His passion, he exclaims, saying: “As a sheep He is led to the slaughter; and as a lamb before his shearer is dumb, so He opened not His mouth in His humility.” Him, moreover, when he described the blows and stripes of His scourgings: “By His bruises we were healed.” Or His humiliation: “And we saw Him, and He had neither form nor comeliness, a man in suffering, and who knows how to bear infirmity.” Or that the people would not believe in Him: “All day long I have spread out my hands unto a people that believes not.” Or that He would rise again from the dead: “And in that day there shall be a root of Jesse, and one who shall rise to reign over the nations; on Him shall the nations hope, and His rest shall be honour.” Or when he speaks of the time of the resurrection: “We shall find Him, as it were, prepared in the morning.” Or that He should sit at the right hand of the Father: “The Lord said unto my Lord, Sit at my right hand, until I shall place Your enemies as the stool of Your feet.” Or when He is set forth as possessor of all things: “Ask of me, and I will give You the heathen for Your inheritance, and the boundaries of the earth for Your possession.” Or when He is shown as Judge of all: “O God, give the King Your judgment, and Your righteousness to the King's Son.” And I shall not in this place pursue the subject further: the things which are announced of Christ are known to all heretics, but are even better known to those who hold the truth.
Source: Treatise Concerning the Trinity (New Advent)