IV. The blessings of the Incarnation stretch backwards as well as reach forward
Accordingly let those men cease their complaints who with disloyal murmurs speak against the dispensations of God, and babble about the lateness of the Lord's Nativity as if that, which was fulfilled in the last age of the world, had no bearing upon the times that are past. For the Incarnation of the Word did but contribute to the doing of that which was done: and the mystery of man's salvation was never in the remotest age at a standstill. What the apostles foretold, that the prophets announced: nor was that fulfilled too late which has always been believed.
But the Wisdom and Goodness of God made us more receptive of His call by thus delaying the work which brought salvation: so that what through so many ages had been foretold by many signs, many utterances, and many mysteries, might not be doubtful in these days of the Gospel: and that the Saviour's nativity, which was to exceed all wonders and all the measure of human knowledge, might engender in us a Faith so much the firmer, as the foretelling of it had been ancient and oft-repeated.
And so it was no new counsel, no tardy pity whereby God took thought for men: but from the constitution of the world He ordained one and the same Cause of Salvation for all. For the grace of God, by which the whole body of the saints is ever justified, was augmented, not begun, when Christ was born: and this mystery of God's great love, wherewith the whole world is now filled, was so effectively presignified that those who believed that promise obtained no less than they, who were the actual recipients.
Source: Sermons (New Advent)