Then David said to the Philistine, “You come to me with a sword and with a spear and with a javelin, but I come to you in the name of the Lord of hosts, the God of the armies of Israel, whom you have defied.” (1 Samuel 17:45, ESV)

The Expositor’s Bible Commentary has an interesting point at this verse: ““You” (Goliath; v.45, emphatic pronoun) and “I” (David; emphatic pronoun) possess weapons that belong to totally different realms (cf. 2 Chronicles 32:7-8; 2 Cor 10:3-4).”

Goliath’s weapons were of the physical realm, they were what was expected at the time:  sword, spear, javelin, and body armor for protection.  David had neither sword, spear, javelin, nor even any body armor, what he did have was the Lord on his side, and as we swiftly find out even against all the “modern” weaponry of the time both offensive and defensive, what David had was more than enough. Goliath thought that the battle was going to be waged in the physical realm, David understood that it would really be fought in the spiritual realm.

It is crucial that we understand what David understood:  the conflicts we encounter today as we follow Christ and are obedient to him, have their roots in the spiritual realm, not the physical.  Or as Paul put it:

For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places.” (Ephesians 6:12, ESV)