Though Saul's "Get Him Wed to Get Him Dead" strategy failed, it didn't fail because of David's greatness and Saul ineptness, but because God "was with" David.
One day, every Christian will have to give an account to God for how they used what was entrusted to them in order to be a blessing to others for the gospel.
The writer of Hebrews encourages his readers toward faith in order that they might have the power to do God's will and the perseverance to stand firm in spite of sinful resistance.
Even though David has defeated Goliath and continues to bring victory over the Philistines, Saul's jealousy leads him to seek David's destruction. The account teaches us about both the suffering of the righteous and the dangers of jealousy.
While the story of David and Goliath is a familiar one, many miss the main point: God is our champion, and He will always fight for us according to His promises...no matter the odds.