Albert Barnes Commentary 1 Samuel 17:47

"and that all this assembly may know that Jehovah saveth not with sword and spear: for the battle is Jehovah`s, and he will give you into our hand." — 1 Samuel 17:47 (ASV)

The Lord saves not with sword... Observe the consistent teaching of such passages as 1 Samuel 14:6; Exodus 14:13–18; Judges 7:2, 4, 7; Psalms 44:6, etc., and their practical use to the Church as lessons of trust in God and distrust of ourselves.