1 This Psalm does contain the things which are said to have been done among the old people: but the new and latter people is being admonished, to beware that it be not ungrateful regarding the blessings of God, and provoke His anger against it, whereas it ought to receive His grace....The Title thereof does first move and engage our attention. For it is not without reason inscribed, “Understanding of Asaph:” but it is perchance because these words require a reader who does perceive not the voice which the surface utters, but some inward sense. Secondly, when about to narrate and mention all these things, which seem to need a hearer more than an expounder: “I will open,” he says, “in parables my mouth, I will declare propositions from the beginning.” Who would not herein be awakened out of sleep? Who would dare to hurry over the parables and propositions, reading them as if self-evident, while by their very names they signify that they ought to be sought out with deeper view? For a parable has on the surface thereof the similitude of something: and though it be a Greek word, it is now used as a Latin word. And it is observable, that in parables, those which are called the similitudes of things are compared with things with which we have to do. But propositions, which in Greek are called προβήλματα, are questions having something therein which is to be solved by disputation. What man then would read parables and propositions cursorily? What man would not attend while hearing these words with watchful mind, in order that by understanding he may come by the fruit thereof?
2. “Hearken ye,” He says, “My people, to My law”. Whom may we suppose to be here speaking, but God? For it was Himself that gave a law to His people, whom when delivered out of Egypt He gathered together, the which gathering together is properly named a Synagogue, which the word Asaph is interpreted to signify. Hath it then been said, “Understanding of Asaph,” in the sense that Asaph himself has understood; or must it be figuratively understood, in the sense that the same Synagogue, that is, the same people, has understood, unto whom is said, “Hearken, My people, unto My law”? Why is it then that He is rebuking the same people by the mouth of the Prophet, saying, “But Israel has not known Me, and My people has not understood”? But, in fact, there were even in that people they that understood, having the faith which was afterwards revealed, not pertaining to the letter of the law, but the grace of the Spirit. For they cannot have been without the same faith, who were able to foresee and foretell the revelation thereof that should be in Christ, inasmuch as even those old Sacraments were significants of those that should be. Had the prophets alone this faith, and not the people too? Nay indeed, but even they that faithfully heard the Prophets, were aided by the same grace in order that they might understand what they heard. But without doubt the mystery of the Kingdom of Heaven was veiled in the Old Testament, which in the fullness of time should be unveiled in the New. “For,” says the Apostle, “they did drink of the Spiritual Rock following them, but the Rock was Christ.” In a mystery therefore theirs was the same meat and drink as ours, but in signification the same, not in form; because the same Christ was Himself figured to them in a Rock, manifested to us in the Flesh. “But,” he says, “not in all of them God was well pleased.” All indeed ate the same spiritual meat and drank the same spiritual drink, that is to say, signifying something spiritual: but not in all of them was God well pleased. When, he says, “not in all:” there were evidently there some in whom was God well pleased; and although all the Sacraments were common, grace, which is the virtue of the Sacraments, was not common to all. Just as in our times, now that the faith has been revealed, which then was veiled, to all men that have been baptized in the name of the Father and of the Son and of the Holy Ghost, the Laver of regeneration is common; but the very grace whereof these same are the Sacraments, whereby the members of the Body of Christ are to reign together with their Head, is not common to all. For even heretics have the same Baptism, and false brethren too, in the communion of the Catholic name.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)