For thou art the God of my strength,.... Who being the strong and mighty God was able to deliver and save him, as well as to plead his cause; and was the author and giver of strength, natural and spiritual, to him; and was the strength of his heart, life and salvation; and is a good reason why he committed his cause unto him; why doest thou cast me off? this is the language of unbelief: it being what was not in reality, only in appearance: the psalmist was ready to conclude he was cast off and rejected of God, because he was afflicted and left in a desolate condition by him, and he did not immediately arise to his help and deliverance, and had withdrawn the light of his countenance from him; but God does not cast off or reject any of his people; they always continue in his love, and in his covenant, and in the hands of his Son; they are always in his sight and family, and shall never perish eternally; and whoever casts them off, or casts them out, he will not; why go I mourning because of the oppression of the enemy? See Gill on Psa 42:9.