Goliath of the Philistines steps forward. He is 10 (or 9, or 6) feet tall, head protected by a bronze helmet, body by a coat of mail, legs by sheaves. He's carrying a javelin and holding a shield.
He hollers out sarcastically, "Why are you positioned to fight a whole army? I'm one man. Just send out one man to fight me. If he beats me, the war is over. You win. If I beat him, the war is over. You lose."
All of Israel, including their king, Saul, hear this challenge. They don't have any 10-foot tall, armor-bearing soldiers to respond to this challenge. They are dismayed. They are afraid.
In the meantime, not even on the battlefield, was David. His brothers are soldiers. He helps his dad take care of the sheep--when he's not playing music for Saul (16:14-23). He arrives at the battlefield in time to hear Goliath repeat his challenge.
Israel will continue to face powerful foes. They will continue to need someone to step forward. As do we.