3 “O God, convert us.” For averse we have been from You, and unless You convert us, we shall not be converted. “And illumine Your face, and we shall be saved”. Hath He anywise a darkened face? He has not a darkened face, but He placed before it a cloud of flesh, and as it were a veil of weakness; and when He hung on the tree, He was not thought the Same as He was after to be acknowledged when He was sitting in Heaven. For thus it has come to pass. Christ present on the earth, and doing miracles, Asaph knew not; but when He had died, after that He rose again, and ascended into Heaven, he knew Him. He was pricked to the heart, and he may have spoken also of Him this testimony which now we acknowledge in this Psalm. You covered Your face, and we were sick: illumine Thou the same, and we shall be whole.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)