David was man after God's heart, he Had faith in God Completely thats why he said "he will go to the battle with no sword but with the word of God"
The Word of God is a Sword Hebrews 4:12 "For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a l discerner of the thoughts and intents m of the heart."
Thanks @desmoniac