My flesh and my heart faileth - Flesh and heart here seem to refer to the whole man, body and soul; and the idea is, that his powers of body and mind failed; were spent; were exhausted. This seems to have been said in an “ideal” sense, or by anticipation. He does not mean to say that his strength then had actually failed, but he seems to have placed himself by imagination in the situation where his strength “would” be all gone - in sickness, in weakness, in sorrow, on the bed of death. He asks himself now what would be his strength then - what would be the object of chief interest and love - on what he would rely; and he answers without hesitation, and with entire confidence, that he could rely on God, and that He would be his portion forever. Even then, when heart and flesh should fail, when all the powers of mind and body should be exhausted, the love of God would survive, and he would find strength and joy in Him.
But God is the strength of my heart - Margin, as in Hebrew, “rock;” the rock on which my heart relies; that is, my refuge, my defense. See the notes at Psalms 18:2. Compare Psalms 61:2.
And my portion for ever - The source of my happiness. Not wealth, then; not honor; not earthly friends; not fame - will be my reliance and the ground of my hope; but that which I shall regard as most valuable - my supreme joy and rejoicing - will be the fact that God is my friend and portion. With all the doubts which I have had in regard to the rectitude of his government, I am sure that when I come to die, I shall cling to him as my hope, my joy, my all. My last refuge - my sufficient refuge - is God. When people come to die, they have “no other refuge” but God. Nothing that they can accumulate of this world’s goods will meet their needs then, for God only can give strength and comfort on the bed of death. Of each and all, however vigorous they may now be, it will be true that “flesh and heart” will “fail;” of each and all it is true that when this shall occur, none but God can be the portion and the strength of the soul.