5 But you will say, the good will of a friend towards me, although I cannot see it, yet can I trace it out by many proofs; but you, what things you will us to believe not being seen, you have no proofs whereby to show them. In the mean time it is no slight thing, that you confess that by reason of the clearness of certain proofs, some things, even such as are not seen, ought to be believed: for even thus it is agreed, that not all things which are not seen, are not to be believed; and that saying, “that we ought not to believe things which we see not,” falls to the ground, cast away, and refuted.
But they are much deceived, who think that we believe in Christ without any proofs concerning Christ. For what are there clearer proofs than those things, which we now see to have been foretold and fulfilled? Wherefore do ye, who think that there are no proofs why ye ought to believe concerning Christ those things which you have not seen, give heed to what things ye see. The Church herself addresses you out of the mouth of a mother's love: “I, whom you view with wonder throughout the whole world, bearing fruit and increasing, was not once such as you now behold me.”
But, “In your Seed shall all nations be blessed.” When God blessed Abraham, He gave the promise of me; for throughout all nations in the blessing of Christ am I shed abroad. That Christ is the Seed of Abraham, the order of successive generations bears witness. Shortly to sum up which, Abraham begot Isaac, Isaac begot Jacob, Jacob begot twelve sons, of whom sprung the people Israel. For Jacob himself was called Israel. Among these twelve sons he begot Judah, whence the Jews have their name, of whom was born the Virgin Mary, who bore Christ.
And, lo, in Christ, that is, in the seed of Abraham, that all the nations are blessed, you see and are amazed: and do ye still fear to believe in Him, in Whom ye ought rather to have feared not to believe? What? doubt ye, or refuse ye to believe, the travail of a Virgin, whereas ye ought rather to believe that it was fitting that so God should be born Man. For this also receive ye to have been foretold by the Prophet; “Behold, a Virgin shall conceive in the womb, and shall bring forth a Son, and they shall call His Name Emmanuel, which is, being interpreted, God with us.”
You will not therefore doubt of a Virgin bringing forth, if you be willing to believe of a God being born; leaving not the governance of the world, and coming unto men in the flesh; unto His Mother bringing fruitfulness, not taking away maidenhood. For thus behooved it that He should be born as Man, albeit He was ever God, by which birth He might become a God unto us. Hence again the Prophet says concerning Him, “Your Throne, O God, is for ever and ever; a sceptre of right, the sceptre of Your Kingdom.
You have loved righteousness, and hated iniquity; therefore God, Your God, has anointed You with the oil of gladness above Your fellows.” This anointing is spiritual, wherewith God anointed God, the Father, that is, the Son: whence called from the “Chrism,” that is, from the anointing, we know Him as Christ. I am the Church, concerning whom it is said unto Him in the same Psalm, and what was future foretold as already done; “There stood at Your right hand the Queen, in a vesture of gold, in raiment of various colors;” that is, in the mystery of wisdom, “adorned with various tongues.”
There it said unto me, Hearken, O daughter, and see, and incline your ear, and forget your own people and your father's house: for the King has desired your beauty: seeing that He is the Lord your God: and the daughters of Tyre shall worship Him with gifts, your face shall all the rich of the people entreat. All the glory of that King's daughter is within, in fringes of gold, with raiment of various colors. There shall be brought unto the King the maidens after her; her companions shall be brought unto You.
They shall be brought with joy and gladness, they shall be brought into the Temple of the King. Instead of your fathers, there are born unto you sons, you shall set them as princes over the whole earth. They shall be mindful of your name, even from generation to generation. Therefore shall the people confess unto you for ever, and for ever and ever.
Source: Concerning Faith of Things Not Seen (New Advent)