His time for making promises was from the days of the prophets until the coming of John the Baptist.
His time for fulfilling them was from then until the end of the world. God is faithful and he has put himself in our debt, not by receiving anything from us but by promising so much.
Nor was a promise sufficient for him; he even bound himself in writing, giving us as it were a pledge in his own hand.
He wanted us to see from Scripture, when the time for fulfilment came, how he was carrying out his promises one by one.
God promised us eternal salvation, everlasting bliss with the angels, an incorruptible inheritance, endless glory, the joyful vision of his face, his holy dwelling in heaven, and after the resurrection from the dead no further fear of dying.
This is what he holds out to us at the end as the goal of all our striving. When we reach it we shall ask for nothing more. But as to how we are to reach our final goal, he revealed this too by promises and prophecies.
God promised men divinity, mortals immortality, sinners justification, outcasts glory.
But because his promise that we who are mortal, corruptible, weak and of low estate, mere dust and ashes, were to be equal to the angels seemed incredible, God not only made a written covenant with us to win our faith, but he also gave us a mediator of his pledge.
This mediator was not a prince, an angel, or an archangel, but his only Son; through his own Son he meant both to show us and give us the way by which he would lead us to the promised goal.
He was not satisfied with sending his Son to show us the way. He made him the way itself. God’s only Son, then, was to come among us, take our human nature, and in this nature be born as a man.
He was to die, to rise again, to ascend into heaven, to sit at the right hand of the Father, and to fulfil his promises among the nations.
After that he was also to fulfil his promise to come again, to demand what he had previously requested, to separate those deserving his anger from those deserving his mercy, to give the wicked what he had threatened and the just what he had promised.
All this had to be prophesied, foretold, and impressed on us as an event in the future so that we should not be terrified by its happening unexpectedly, but wait for it with faith.
Augustine of Hippo (354-430): Commentary on Psalm 109, 1-3 (CSEL 40:1601-1603); from the Monastic Office of Vigils, Thursday of the 2nd Week in Advent, Year 1.