St Augustine’s commentary on psalm 109
God’s promises are given to us through the Son
God decreed a time for making promises and a time for the promises to be fulfilled. The time for making promises was the time of the prophets, ending with John the Baptist, the last prophet. From then until the end is the time for the fulfilment of promises.
God is faithful. He has made himself our debtor, not by receiving anything from us but by promising us so much. The promise alone was not enough for him: he wanted it in writing, so that he could be held to it, practically entering into a contract with us that listed the promises he was making. In that way, when he began to fulfil his promises, we could see the order of their fulfilment by looking in Scripture. Therefore the time of the prophets was (as I have said so often) the time of making promises.
He promised us eternal salvation and an unending life of blessedness with the angels, and an imperishable inheritance, the joy of seeing his face, a dwelling-place with him in heaven, and the fear of death removed from us through the resurrection. This is, if you like, his ultimate promise. We look forward to it, and when we reach it, we will want nothing more. But as to how this final end is to be reached, he has also told us in promises and prophecies.
He has promised to men that they will be like God; to mortals he has promised immortality; to sinners, righteousness; to the lowly, glory.
Indeed, brethren, because what God promised seemed incredible to men – that from mortality, decay, weakness, lowliness, dust and ashes they should become equals of the angels of God – he did not only sign a contract with them to convince them. He sent, not just any prince, not just any angel or archangel, but his only Son. The road by which he was to lead us to the end he had promised us – through his Son he would show us that road.
Even so, it was not enough for God to send his Son to point out the way – he made his Son the way itself, so that we can go on our journey guided by him as he walks along his own way.
So the only Son of God was to come to men, to take on humanity, and thus to die, to ascend to heaven and sit at the right hand of the father, and so to fulfil what he had promised among the nations. After that promise to the nations had been fulfilled, he would fulfil his other promise, to come, to demand the return of what he had given, to separate the vessels of anger from the vessels of mercy, to give the wicked what he had threatened and the righteous what he had promised.
All this had to be prophesied and foretold. It had to have its coming announced. It could not come suddenly and unexpectedly, causing terror and alarm: people had to be awaiting it with faith.