Basil of Caesarea

One does not sing to the Lord by simply uttering the words of the psalm with his mouth, but all who send up the psalmody from a clean heart and who are holy, maintaining righteousness toward God, these are able to sing to God, harmoniously guided by the spiritual rhythms. How many stand there, coming from fornication? How many from theft? How many concealing in their hearts deceit? How many lying? They think they are singing, although in truth they are not singing. For the Scripture invites the saint to the singing of psalms. …

HOMILIES on the PSALMS 14:3 (PS 30)

Augustine of Hippo

"Sing to the Lord, O you saints of His." The prophet seeing these future things, rejoices, and says, "Sing to the Lord, O you saints of His. And make confession of the remembrance of His holiness" [Psalm 30:4]. And make confession to Him, that He has not forgotten the sanctification, wherewith He has sanctified you, although all this intermediate period belong to your desires.

Exposition on Psalm 30 ↗

Thomas Aquinas

Then when he says, "Sing to the Lord, you his saints," etc., he induces others first to give thanks. Second he commemorates the benefits, at "For there is wrath in his indignation." Where he does two things: first he shows who those are that owe thanks to God. Second, for what, at "Give thanks." He says therefore: "Sing." Behold how it is a Psalm of a canticle. "Sing," I say, "you saints": because Sir. 15: "Praise is not seemly in the mouth of a sinner": Rev. 19: "Give praise to our God, all you his saints." But for what? …

Exposition on the Psalms of David ↗