1 The title of it is, “Unto the end, in behalf of Idithun, a Psalm to David himself.” I recollect that already to you has been explained what Idithun is....Let us see how far he has leaped over, and whom he has “leaped over,” and in what place, though he has leaped over certain men, he is situate, whence as from a kind of spiritual and secure position he may behold what is below....He being set, I say, in a certain fortified place, does say, “Shall not my soul be subject to God?”. For he had heard, “He that does exalt himself shall be humbled; and he that humbles himself shall be exalted:” and fearful lest by leaping over he should be proud, not elated by those things which were below, but humble because of Him that was above; to envious men, as it were threatening to him a fall, who were grieved that he had leaped over, he has made answer, “Shall not my soul be subject to God?”...“For from Himself is my salvation.” “For Himself is my God and my salvation, my taker up, I shall not be moved more”. I know who is above me, I know who stretches forth His mercy to men that know Him, I know under the coverings of whose wings I should hope: “I shall not be moved more.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)