33 These promises, so sure, so firm, so open, so unquestioned, were made concerning Christ. For although some are mysteriously veiled, yet some are so clear, that all that is obscure is easily revealed by them. Such being the case, see what follows: “But You have approved and brought to nothing and forsaken Your Anointed”. “You have overthrown the testament of Your servant, and profaned His holiness on the ground”. “You have broken down all His hedges, and made His strongholds a terror”....How is this?
You have promised all those things: and You have brought to pass their reverse. Where are now the promises which but a little before filled us with delight? Which we so joyfully applauded, which we so fearlessly made our boast of? It is as if one promised, and another destroyed. And this is the mystery: for the words are not “another,” but “You,” Thou who promised, who even swore in condescension to human doubt, You have promised this, and done thus! Whence shall I get Your oath, where shall I find Your promise fulfilled?
Would then God promise, or swear thus falsely? And yet why then these promises, and these acts? I answer, that He acted thus in fulfilment of those promises. But who am I, to say this? Let us see therefore whether it is the language of the Truth; what I say will not then be without foundation. It was David to whom the fulfilment of these promises in his seed, that is, in Christ, was promised: and as they were addressed to David, men expected their completion in David. Further, lest when any Christian asserted these promises to have referred to Christ, another by applying them to David, because he described the fulfilment of all of them in David, might thus err; He cancelled them in David, thus obliging us when we see them unfulfilled in David, to look to another quarter for their fulfilment.
Thus also in the case of Esau and Jacob, we find the elder worshipped by the younger, though it is written, “The elder shall serve the younger;” so when you see it unfulfilled in those two brothers, you look for two peoples in whom to discover the completion of what God in His truth deigns to promise. “From the fruit of your body,” says the Lord unto David, “shall I set upon your sea.” He promised from his seed something for evermore: and, Solomon, born to him, became master of such wisdom, that the promise of God respecting the fruit of David's body was believed to have been fulfilled in him; but Solomon fell, and gave room for hoping for Christ; that since God can neither be deceived nor deceive, He might not make His promise to rest in one who He knew would fall, but you might after the fall of Solomon look back to God, and demand His promise.
Have You, O Lord, deceived? Have You failed to fulfil Your promise? Do You not exhibit what You have sworn? Perhaps God might reply, I swore and promised: but Solomon would not persevere. What then? Did not You, Lord God, know beforehand that he would not persevere? Indeed You knew. Why then did You promise me what should be eternal in one who would not persevere? Have You not answered; “But if his children forsake My law, and walk not in My judgments; if they keep not My statutes, and profane My testament;” yet My promise shall remain, and My oath shall be fulfilled: “I have sworn once in My Holiness,” within, in a certain mystery, in the very spring whence the Prophets drank, whence they burst forth to us of these things, “I have sworn once” that I will not fail David.
Show forth then what You have sworn, give us what You have promised. The fulfilment is taken from that David, that it might not be looked for in that David: wait therefore for what I have promised.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)