4 “Because I will pray unto You. O Lord, in the morning You will hear my voice.” What does that, which he said above, “Hear Thou,” mean, as if he desired to be heard immediately? But now he says, “in the morning You will hear;” not, hear Thou: and, “I will pray unto You;” not, I do pray unto You: and, as follows, “in the morning I will stand by You, and will see;” not, I do stand by You, and do see. Unless perhaps his former prayer marks the invocation itself: but being in darkness amidst the storms of this world, he perceives that he does not see what he desires, and yet does not cease to hope, “For hope that is seen, is not hope.” Nevertheless, he understands why he does not see, because the night is not yet past, that is, the darkness which our sins have merited.
He says therefore, “Because I will pray unto You, O Lord;” that is, because You are so mighty to whom I shall make my prayer, “in the morning You will hear my voice.” You are not He, he says, that can be seen by those, from whose eyes the night of sins is not yet withdrawn: when the night then of my error is past, and the darkness gone, which by my sins I have brought upon myself, then “You will hear my voice.” Why then did he say above not, “You will hear,” but “hear Thou”? Is it that after the Church cried out, “hear Thou,” and was not heard, she perceived what must needs pass away to enable her to be heard?
Or is it that she was heard above, but does not yet understand that she was heard, because she does not yet see by whom she has been heard; and what she now says, “In the morning You will hear,” she would have thus taken, In the morning I shall understand that I have been heard? Such is that expression, “Arise, O Lord,” that is, make me arise. But this latter is taken of Christ's resurrection: but at all events that Scripture, “The Lord your God proves you, that He may know whether ye love Him,” cannot be taken in any other sense, than, that you by Him may know, and that it may be made evident to yourselves, what progress you have made in His love.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)