For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Joe cattani
What I take away from David and Goliath this; God don't call the qualified, He qualifies the called.
Corrie Lof
It also shows God's victory over Satan. Satan tries consistently to overthrow God's plan of redemption spoken of in Gen. 3, Satan knows that David is a key to this plan, and tries to crush him time and time again.