VII. Christ's cry of Forsaken on the cross was to teach us the insufficiency of the human nature without the Divine
Hence it is that the Lord Jesus Christ, our Head, representing all the members of His body in Himself, and speaking for those whom He was redeeming in the punishment of the cross, uttered that cry which He had once uttered in the psalm, “O God, My God, look upon Me: why have You forsaken Me?” That cry, dearly-beloved, is a lesson, not a complaint. For since in Christ there is one person of God and man, and He could not have been forsaken by Him, from Whom He could not be separated, it is on behalf of us, trembling and weak ones, that He asks why the flesh that is afraid to suffer has not been heard.
For when the Passion was beginning, to cure and correct our weak fear He had said, “Father, if it be possible, let this cup pass from Me: nevertheless not as I will but as You;” and again, “Father, if this cup cannot pass except I drink it, Your will be done.” As therefore He had conquered the tremblings of the flesh, and had now accepted the Father's will, and trampling all dread of death under foot, was then carrying out the work of His design, why at the very time of His triumph over such a victory does He seek the cause and reason of His being forsaken, that is, not heard, save to show that the feeling which He entertained in excuse of His human fears is quite different from the deliberate choice which, in accordance with the Father's eternal decree, He had made for the reconciliation of the world?
And thus the very cry of “Unheard” is the exposition of a mighty Mystery, because the Redeemer's power would have conferred nothing on mankind if our weakness in Him had obtained what it sought. Let these words dearly-beloved, suffice today, lest we burden you by the length of our discourse: let us put off the rest till Wednesday. The Lord shall hear you if you pray that we may keep our promise through the bounty of Him Who lives and reigns for ever and ever. Amen.
Source: Sermons (New Advent)