Albert Barnes 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 save the Lord? – Who is God except Yahweh? The idea is that no other being has demonstrated the power, the wisdom, and the goodness that properly belong to the true God; or, that the things which are implied in the true nature of God are found in no other being.

Or who is a rock save our God? – See Psalm 18:2. There is no one who can provide such safety or defense; no one under whose protection we can be secure in danger. Compare Deuteronomy 32:31.