1 This Psalm's Title is, “Of the Understanding of Asaph.” Asaph in Latin is translated congregation, in Greek Synagogue. Let us see what this Synagogue has understood. But let us understand firstly Synagogue: from thence we shall understand what the Synagogue has understood. Every congregation is spoken of under the general name of Synagogue: one both of beasts and of men may be called a congregation; but here there is no congregation of beasts when we heard “understanding.”...for this the Psalm's Title does prescribe, saying, “Of the understanding of Asaph.”
It is therefore a certain understanding congregation whereof we are about to hear the voice. But since properly Synagogue is said of the congregation of the people of Israel, so that wheresoever we may have heard Synagogue, we are no longer wont to understand any but the people of the Jews; let us see whether perchance the voice in this Psalm be not of that same people. But of what sort of Jews and of what sort of people of Israel? For they are not of the chaff, but perchance of the grain; not of the broken branches, but perchance of those that are strengthened.
“For not all that are of Israel are Israelites.”...There are therefore certain Israelites, of whom was he concerning whom was said, “Behold an Israelite indeed, in whom guile is not.” I do not say in the same manner as we are Israelites, for we also are the seed of Abraham. For to the Gentiles the Apostle was speaking, when he said, “Therefore the seed of Abraham you are, heirs according to promise.” According to this therefore all we are Israelites, that follow the footsteps of the faith of our father Abraham.
But let us understand here the voice of the Israelites in the same manner as the Apostle says, “For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin.” Here therefore let us understand that whereof the Prophets have spoken, “a remnant shall be saved.” Of the remnant therefore saved let us hear in this place the voice; in order that there may speak that Synagogue which had received the Old Testament, and was intent upon carnal promises; and by this means it came to pass that their feet were shaken.
For in another Psalm, where too the title has Asaph, there is said what? “How good is the God of Israel to men right in heart. But my feet were almost moved.” And as if we were saying, whence were your feet moved? “Well near,” he says, “my steps were overthrown, because I was jealous in the case of sinners, looking on the peace of sinners.” For while according to the promises of God belonging to the Old Testament he was looking for earthly felicity, he observed it to abound with ungodly men; that they who worshipped not God were enriched with those things which he was looking for from God: and as though without cause he had served God, his feet tottered....But opportunely it has chanced not by our own but by God's dispensation, that just now we heard out of the Gospel, that “the Law was given by Moses, Grace and Truth came by Jesus Christ.” For if we distinguish between the two Testaments, Old and New, there are not the same Sacraments nor the same promises; nevertheless, the same commandments for the most part....When examined they are either all found to be the same, or there are scarce any in the Gospel which have not been spoken by the Prophets. The Commandments are the same, the Sacraments are not the same, the Promises are not the same. Let us see wherefore the commandments are the same; because according to these we ought to serve God. The Sacraments are not the same, for some Sacraments there are giving Salvation, others promising a Saviour. The Sacraments of the New Testament give Salvation, the Sacraments of the Old Testament did promise a Saviour. When therefore you have now the things promised, why do you seek the things promising, having now the Saviour?...God through the New Testament has taken out of the hands of His sons those things which are like the playthings of boys, in order that He might give something more useful to them growing up, on that account must He be supposed not to have given those former things Himself. He gave both Himself. But the Law itself through Moses was given, Grace and Truth came through Jesus Christ: Grace because there is fulfilled through love that which by the letter was being enjoined, Truth because there is being rendered that which was promised. This thing therefore this Asaph has understood. In a word, all things which to the Jews had been promised have been taken away. Where is their kingdom? Where the Temple? Where the Anointing? Where is Priest? Where are now the Prophets among them? From what time there came He that by the Prophets was foretold, in that nation there is now nothing of these things; now she has lost things earthly, and not yet does seek things Heavenly.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)