Sometimes the most well-known stories in the Bible can be the more difficult to interpret and apply BECAUSE they are so familiar. Yet, perhaps we can find something fresh and new here. If not, let's not lose sight of the old spiritual lessons that should be obvious.
Now, in the last chapter we left David standing in a royal place holding Saul's shield... or playing a harp. However, I Samuel 17:15 tells us that David did not stay with Saul. He returned home to take care of his father's sheep. Meanwhile Saul went to war again against an attacking force of Philistines. Among the soldiers on the Philistine side was a strong man who was almost 10 feet tall. Alone, this giant named Goliath struck fear into the hearts of the Jews. But, Goliath was nothing more than an opportunity for God to manifest His power and to catapult David into a prominent position in the nation.
There are many interesting subthemes in this story. David's interaction with his brothers, with Goliath, with Saul... it's all very interesting. How about David's willingness to attempt to go in Saul's armor (which he would have been very familiar with... as Saul’s former armor bearer)? The return of David with Goliath's head in his hand after the battle; there's a mind boggling scene. But what is the most basic lesson to be learned here? Here it is: faith in God and spiritual courage can bring victory over any enemy, no matter what the odds.
David came upon the battle scene innocently. But, when he heard the blasphemy of Goliath and realized that nobody was doing anything about it, he volunteered. And, with just a staff, a sling and some stones, David went out alone to conquer Israel's foe. And, conquer he did. Not only that, but the whole army of Israel took courage after the decapitation of Goliath and so they too went forward into battle (after having remained for many weeks previously in cowardice and fear).