3 Choose for yourself Him, who will neither sleep nor slumber, and your foot shall not be moved. God is never asleep: if you dost wish to have a keeper who never sleeps, choose God for your keeper. “Suffer not my feet to be moved,” you say, well, very well: but He also says unto you, “Let not him that keeps you slumber.” Thou perhaps wast about to turn yourself unto men as your keepers, and to say, whom shall I find who will not sleep? What man will not slumber? Whom do I find?
Whither shall I go? Whither shall I return? The Psalmist tells you: “He that keeps Israel, shall neither slumber nor sleep”. Do you wish to have a keeper who neither slumbers nor sleeps? Behold, “He that keeps Israel shall neither slumber nor sleep:” for Christ keeps Israel. Be thou then Israel. What means Israel? It is interpreted, Seeing God. And how is God seen? First by faith: afterwards by sight. If you can not as yet see Him by sight, see Him by faith...Who is there, who will neither slumber nor sleep?
When you seek among men, you are deceived; you will never find one. Trust not then in any man: every man slumbers, and will sleep. When does he slumber? When he bears the flesh of weakness. When will he sleep? When he is dead. Trust not then in man. A mortal may slumber, he sleeps in death. Seek not a keeper among men.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)