Thomas Aquinas, on Ps 33:1

“For David, when he changed his countenance before Achimelech, who dismissed him, and he went his way. [1 Kings 21]”

“The title: "A Psalm of David, when he changed his countenance before Abimelech, and he dismissed him, and he departed." This history is found in 1 Sam. 21, where it is said that David, fleeing from the face of Saul, came to Achish the king of Gath, and was recognized there, along with his valor, because he had slain the Philistine. And fearing that danger threatened him on this account, because those men were of the race of the Philistines and also because of envy of his valor, he wished to avoid this and feigned himself a fool, and so that king despised him. All this is found in 1 Sam. 21, except that the name does not agree, because there the king is called Achish, but here Abimelech. Nor is this inconsistent, either because he had two names, or because his name was Achish but he was of the family of Abimelech. Hence he changed his countenance by showing himself to be a fool, and the king dismissed him and he departed, because David, cast out by him, departed and withdrew. Mystically, Christ changed his countenance when he changed his sacrament, in which the divine truth was concealed. Or, Christ changed the old paschal sacrament into the new before Abimelech, which is interpreted "the kingdom of my father." The Father of Christ is God according to his divinity, and David according to his humanity. The kingdom of David is the Jewish people; the kingdom of God is the Church. Christ indeed changed his countenance before Abimelech, that is, before the Jews, because they were the kingdom of his father David, and they did not recognize him: Is. 53: "We saw him, and there was no comeliness"; and they despised him: hence, "nor did we esteem him." And he departed to the Gentiles. Or Achish, which is interpreted "incredulous," signifies the Jews. In the preceding Psalm, the Psalmist set forth the dignity of the just; here, however, he invites others to the praise of God. This Psalm is divided into two parts. First, an exhortation to praise is presented. Second, a certain necessary instruction, at "Come, children." Concerning the first he does two things. First, he treats of the praise of God. Second, he presents the matter of praise, at "I sought." Concerning the first he does two things. First, he sets forth the example of praise. Second, he exhorts others to imitate, at "Let the meek hear." Concerning the first he does two things. First, he presents the example of praise in himself. Second, the fruit of praising God, at "In the Lord my soul shall be praised." He says therefore, "I will bless the Lord." Sometimes a person praises God for his own sake, as when he speaks with his tongue, he speaks only to himself. Sometimes for the consolation of others, as when he prophesies also to others. To bless the Lord, therefore, is, as has been said, to confess the praise of God; but to bless the Lord is to do good: "at all times," namely of adversity and prosperity. Against this, Ps. 48: "He will praise you when you do good for him"; but Job did not act thus, Job 2: "If we have received good things from the hand of the Lord, shall we not also accept evil things?" etc. Tob. 4: "At all times bless God." Likewise, it is necessary that one not only bless God in himself, namely in his own heart, but also that he have his praise on his lips. For the necessity of vocal praise is that you not only praise God yourself, but also that for the benefit and encouragement of others you praise him. Hence he says, "His praise is always in my mouth": Is. 51: "Joy and gladness shall be found in it, thanksgiving and the voice of praise." "Always," however, he says, that is, at every appointed time. Or, in the preparation of the mind. Or, always by doing good, from which God is always praised. Note that this verse is chanted at the sixth hour, when Christ suffered, whose passion is for us a cause of praise.”

Imported from an open dataset — not yet checked against the printed edition.