Psalms 89:18 - Albert Barnes' Notes on the Bible

For the Lord is our defense - Margin, “Our shield is of the Lord.” The original word rendered “defense,” is shield. Compare Psalms 5:12, note; Psalms 33:20, note; Psalms 59:11, note. The meaning is, that protection was to be found in God alone. The true construction of this verse is, “For to Yahweh (belongs) our shield, and to the Holy One of Israel our king.” That is, All that they had, and all that they relied on as a defense, belonged to God, or was of God; in other words, their very protectors were themselves protected by Yahweh. They had no other defense; nothing else on which they could depend.