For thou art great, and doest wondrous things - Things suited to excite wonder or admiration; things which lie beyond the power of any creature, and which could be performed by no one but a being of almighty power. A God who could do these things could also do that which the psalmist asked of him, for what God actually does proves that there is nothing within the limits of possibility which he cannot perform. The greatness and the power of God are reasons why we should appeal to him in our weakness, and in our times of trouble.
Thou art God alone - Thou only canst do what a God can do, or what belongs to God. In those things, therefore, which require the interposition of divine power our appeal must be to thee alone. So in the matter of salvation.