I. The Incarnation fulfils all its types and promises
The Divine goodness, dearly beloved, has indeed always taken thought for mankind in various manners, and in many portions, and of His mercy has imparted many gifts of His providence to the ages of old; but in these last times has exceeded all the abundance of His usual kindness, when in Christ the very Mercy has descended to sinners, the very Truth to those that are astray, the very Life to those that are dead: so that Word, which is co-eternal and co-equal with the Father, might take our humble nature into union with His Godhead, and, being born God of God, might also be born Man of man.
This was indeed promised from the foundation of the world, and had always been prophesied by many intimations of facts and words: but how small a portion of mankind would these types and fore-shadowed mysteries have saved, had not the coming of Christ fulfilled those long and secret promises: and had not that which then benefited but a few believers in the prospect, now benefited myriads of the faithful in its accomplishment. Now no longer then are we led to believe by signs and types, but being confirmed by the gospel story we worship that which we believe to have been done; the prophetic lore assisting our knowledge, so that we have no manner of doubt about that which we know to have been predicted by such sure oracles.
For hence it is that the Lord says to Abraham: “In your seed shall all nations be blessed:” hence David, in the spirit of prophecy, sings, saying: “The Lord swore truth to David, and He shall not frustrate it: of the fruit of your loins will I set upon your seat;” hence the Lord again says through Isaiah: “behold a virgin shall conceive in her womb, and shall bear a Son, and His Name shall be called Emmanuel, which is interpreted, God with us,” and again, “a rod shall come forth from the root of Jesse, and a flower shall arise from his root.” In which rod, no doubt the blessed Virgin Mary is predicted, who sprung from the stock of Jesse and David and fecundated by the Holy Ghost, brought forth a new flower of human flesh, becoming a virgin-mother.
Source: Sermons (New Advent)