2 This Psalm is entitled, a Psalm to be sung on the Sabbath day. Lo, this day is the Sabbath, which the Jews at this period observe by a kind of bodily rest, languid and luxurious. They abstain from labours, and give themselves up to trifles; and though God ordained the Sabbath, they spend it in actions which God forbids. Our rest is from evil works, theirs from good; for it is better to plough than to dance. They abstain from good, but not from trifling, works. God proclaims to us a Sabbath. What sort of Sabbath? First consider, where it is. It is in the heart, within us; for many are idle with their limbs, while they are disturbed in conscience....That very joy in the tranquillity of our hope, is our Sabbath. This is the subject of praise and of song in this Psalm, how a Christian man is in the Sabbath of his own heart, that is, in the quiet, tranquillity, and serenity of his conscience, undisturbed; hence he tells us here, whence men are wont to be disturbed, and he teaches you to keep Sabbath in your own heart.
3....Accuse yourself, and you receive indulgence. Besides, many do not accuse Satan but their fate. My fate led me, says one when you ask him, why did you do it? Why did you sin? He replies, by my evil fate. Lest he should say, I did it; he points to God as the source of his sin: with his tongue he blasphemes. He says not this indeed openly as yet, but listen, and see that he says this. You ask of him, what is fate: and he replies, evil stars. You ask, who made, who appointed the stars; he can only answer, God. It follows, then, that whether he does so directly or indirectly, still he accuses God, and when God punishes sins, he makes God the author of his own sins. It cannot be that God punishes what He has wrought: He punishes what you do, that He may set free what He has wrought. But sometimes, setting aside everything else, they attack God directly: and when they sin, they say, God willed this; if God had not willed it, I should not have sinned. Does He warn you for this, that not only He may not be listened to, to keep you from sin, but even be accused because you sin? What then does this Psalm teach us? “It is a good thing to confess unto the Lord.” What is to confess unto the Lord? In both cases: both in your sins, because you have done them; and in your good works, confess unto the Lord, because He has done them. Then shall you “sing unto the Name of God, the Most Highest:” seeking the glory of God, not your own; His Name, not yours. For if you seek the Name of God, He also seeks your name; but if you have neglected the Name of God, He also does blot out yours....
4. “To tell of Your mercy early in the morning, and of Your truth in the night season”. What is the meaning of this; that the mercy of God is to be told us in the morning, and in the night the truth of God? The morning is, when it is well with us; the night, the sadness of tribulation. What then did he say in brief? When you are prosperous, rejoice in God, for it is His mercy. Now, perhaps you would say, If I rejoice in God, when I am prosperous, because it is His mercy; what am I to do when I am in sorrow, in tribulation? It is His mercy, when I am prosperous; is it then His cruelty, when I am in adversity? If I praise His mercy when it is well with me, am I then to exclaim against His cruelty when it is ill? No. But when it is well, praise His mercy: when ill, praise His truth: because He scourges sins, He is not unjust....During the night Daniel confessed the truth of God: he said in his prayer, “We have sinned, and committed iniquity, and have done wickedly. O Lord, righteousness belongs unto You: but unto us confusion of face.” He told of the truth of God during the night-season. What is it to tell of the truth of God in the night-season? Not to accuse God, because you suffer anything of evil: but to attribute it to your sins, His correction: to tell of His loving-kindness early in the morning, and of His truth in the night-season. When you do this, you always praise God, always confess to God, and sing unto His Name.
5. “Upon a psaltery of ten strings, with a song, and upon the harp”. You have not heard of the psaltery of ten strings for the first time: it signifies the ten commandments of the Law. But we must sing upon that psaltery, and not carry it only. For even the Jews have the Law: but they carry it: they sing not....“And upon the harp.” This means, in word and deed; “with a song,” in word; “upon the harp,” in work. If you speak words alone, you have, as it were, the song only, and not the harp: if you work, and speakest not, you have the harp only. On this account both speak well and do well, if you would have the song together with the harp.
6. “For Thou, Lord, hast made me glad through Your works; and I will rejoice in giving praise for the operations of Your hands”. You see what he says. You have made me living well, You have formed me: if by chance I do anything of good, I will rejoice in the work of Your hands: as the Apostle says, “For we are His workmanship, created unto good works.” For unless He formed you to good works, you would not know any works but evil....Because you can not have truth from your own self, it remains that thou drink it thence, whence it flows: as if you have gone back from the light, you are in darkness: as a stone glows not with its own heat, but either from the sun or fire, and if you withdraw it from the heat, it cools: there it appears, that the heat was not its own; for it became heated either by the sun or by fire: thus thou also, if you withdraw from God, wilt become cold; if you approach God, you will warm: as the Apostle says “fervent in spirit.” Also what says he of the light? If you approach Him, you will be in light; therefore says the Psalm, “Look upon Him, and be lightened; and your faces shall not be ashamed.” Because therefore you can do no good, unless lightened by the light of God, and warmed by the spirit of God; when you shall see yourself working well, confess unto God, and say what the Apostle says; say unto yourself, that you be not puffed up, “For what have you that thou did not receive?”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)