6 “His truth shall surround you with a shield”. What are “the wings,” the same is “the shield:” since there are neither wings nor shield. If either were literally, how could the one be the same as the other? Can wings be a shield or a shield wings? But all these expressions, indeed, are figuratively used through likenesses. If Christ were really a Stone, He could not be a Lion; if a Lion, He could not be a Lamb: but He is called both Lion, and Lamb, and Stone, and Calf, and anything else of the sort, metaphorically, because He is neither Stone, nor Lion, nor Lamb, nor Calf, but Jesus Christ, the Saviour of all of us, for these are likenesses, not literal names.
“His truth shall be your shield,” it is said: a shield to assure us that He will not confound those whose trust is in themselves with those who hope in God. One is a sinner, and the other a sinner: but suppose one that presumes upon himself is a despiser, confesses not his sins, and he will say, if my sins displeased God, He would not suffer me to live. But another dared not even raise his eyes, but beat upon his breast, saying, “God be merciful to me a sinner.” Both this was a sinner, and that: but the one mocked, the other mourned: the one was a despiser, the other a confessor, of his sins.
But the truth of God, which respects not persons, discerns the penitent from him who denies his sin, the humble from the proud, him who presumes upon himself from him who presumes on God. “You shall not be afraid for any terror by night.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)