Albert Barnes Commentary 1 Samuel 17:26

"And David spake to the men that stood by him, saying, What shall be done to the man that killeth this Philistine, and taketh away the reproach from Israel? for who is this uncircumcised Philistine, that he should defy the armies of the living God?" — 1 Samuel 17:26 (ASV)

The living God - This fine expression occurs first in Deuteronomy, and then in Joshua 3:10 and 2 Kings 19:4. We find it twice in the Psalms of David (Psalms 42:2; Psalms 84:2), four times in the prophets, and frequently in the New Testament. It is generally used in contrast to false gods (for example, 1 Thessalonians 1:9).