John Calvin Commentary Psalms 18:31

"For who is God, save Jehovah? And who is a rock, besides our God," — Psalms 18:31 (ASV)

For who is God besides Jehovah? David here, deriding the foolish inventions of men, who, according to their own fancy, make for themselves guardian gods, confirms what I have said before: that he never undertook anything except by the authority and command of God. If he had passed beyond the limits of his calling, he could not have said with such confidence that God was on his side.

Besides, although in these words he sets the true God against all the false gods invented by men, his purpose, at the same time, is to overthrow all the vain hopes in which the world is entangled, and by which it is tossed about and prevented from resting in God.

The question David addresses here is not the mere title and name of God, but he declares that whatever assistance we need we should seek from God, and from nowhere else, because he alone is endowed with power: Who is strong except our God? We should, however, consider David’s purpose, which I mentioned earlier, namely, that by confidently representing God as opposed to all his enemies, and as the leader under whose standard he had valiantly fought against them, he means to affirm that he had attempted nothing according to his own fancy, or with an evil and condemning conscience.