Psalms 89:21

With whom my hand shall be established - Septuagint: “My hand shall aid him.” Luther; “My hand shall hold him.” DeWette; “With him my hand shall be continually.” Professor Alexander; “Shall ever be present.” The idea is, that God would always defend or protect him. He would not merely interpose at times, or at intervals, but he would be his constant protector. His hand would be permanently, or constantly, extended for his aid - as if it were a part of David’s own person, or were his own hand, to be used as he pleased. So God is the constant helper of his people. They may rely on his power; they may avail themselves of it, as if it were their own.

Mine arm also shall strengthen him - In using his own arm, he will in fact make use of the strength of mine. The people of God are as really defended as if the strength of God were theirs; or as if they were themselves almighty. The omnipotence of God is employed in their defense, and it will be as certainly exerted in their favor, and as constantly, as if it were their own. It will be no less surely employed in their defense in the hand of God than if it were in their own hand. It will be more wisely employed by him in their behalf than it would be by themselves.