2 Sing therefore what follows; if you wish to hear how you may most securely set your feet on the steps, so that you may not be fatigued in that ascent, nor stumble and fall: pray in these words: “Suffer not my foot to be moved!”. Whereby are feet moved; whereby was the foot of him who was in Paradise moved? But first consider whereby the feet of him who was among the Angels were moved: who when his feet were moved fell, and from an Angel became a devil: for when his feet were moved he fell.
Seek whereby he fell: he fell through pride. Nothing then moves the feet, save pride: nothing moves the feet to a fall, save pride. Charity moves them to walk and to improve and to ascend; pride moves them to fall...Rightly therefore the Psalmist, hearing how he may ascend and may not fall, prays unto God that he may profit from the vale of misery, and may not fail in the swelling of pride, in these words, “Suffer not my feet to be moved!” And He replies unto him, “Let him that keeps you not sleep.”
Attend, my beloved. It is as if one thought were expressed in two sentences; the man while ascending and singing “the song of degrees,” says, “Suffer not my foot to be moved:” and it is as if God answered, You say unto Me, Let not my feet be moved: say also, “Let Him that keeps you not sleep,” and your foot shall not be moved.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)