2Sa 21:18 Now it happened afterward that there was again a battle with the Philistines at Gob. Then Sibbechai the Hushathite killed Saph, who was one of the sons of the giant.
19 Again there was war at Gob with the Philistines, where Elhanan the son of Jaare-Oregim the Bethlehemite killed the brother of Goliath the Gittite, the shaft of whose spear was like a weaver's beam.
20 Yet again there was war at Gath, where there was a man of great stature, who had six fingers on each hand and six toes on each foot, twenty-four in number; and he also was born to the giant.
21 So when he defied Israel, Jonathan the son of Shimea, David's brother, killed him.
22 These four were born to the giant in Gath, and fell by the hand of David and by the hand of his servants.
Davis was a giant-killer. The soldiers of King Saul, however, were not. But when David became the leader, other giant-killers emerged. Other mighty men also emerged. When soldiers associated with David, they learned how to be like him. Thus, they learned how to be valiant in battle and kill giants.
Don't waste your Christian life listening to critics and excuse-makers. Find someone who is being used by God and listen to him (or her).