4 “You have said, I have made a covenant with My chosen”. What covenant, but the new, by which we are renewed to a fresh inheritance, in our longing desire and love of which we sing a new song. “I have made a covenant with My chosen,” says the Psalmist: “I have sworn unto David My servant.” How confidently does he speak, who understands, whose mouth serves truth! I speak without fear; since “You have said.” If You make me fearless, because You have said, how much more so dost Thou make me, when You have sworn! For the oath of God is the assurance of a promise. Man is justly forbidden to swear: lest by the habit of swearing, since a man may be deceived, he fall into perjury. God alone swears securely, because He alone is infallible.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)