Fear is contagious and can be powerful. It can be amplified to petrify the entire national army for 40 days. However, those who are rooted in the covenant of the LORD can reject that kind of fear.
Summary
The camp of Israel and Philistines gathered at the opposite side of the Valley. Goliath, a heavily-armored Philistine of more than 9 feet tall, provoked the army of Israel and instilled fear into the heart of Saul and the entire Israelites. The entire Israelite army continued to be petrified for 40 days as the taunt of the Philistine continued.
David came to the camp to provide supplies to his brothers, who were among the armies. He heard the taunting of the Philistine and became indignant.

Contagious Fear
Who was the natural person to lead the Israel army when Goliath, the Philistine, taunted and defied them? That would be Saul himself. Not only was he the king, but he was also a very tall person as well. King Saul may not have been as tall as Goliath, but he was described as a head taller than the rest of the Israelites (1 Sam 10:23).
His leadership greatly mattered. Yet, he fell into fear (17:11), and that was not good. When a leader fell into fear, the effect of fear would amplify to his/her followers. Therefore, the entire Israel greatly feared and was dismayed.
Source of Fear
Although Saul was the legitimate, appropriate choice to fight Goliath one-on-one, he did not have to fight Goliath himself. It would not be rational to send a king to fight one-on-one battle. However, King Saul had no reason to fear Goliath, either.
Then, what was the source of the fear? The primary source of fear was the absence of the Lord God in the heart and mind of Saul and the entire nation of Israel.
As Saul disobeyed the word of the Lord, Saul was rejected being king (15:23). The Spirit of the LORD was no longer with Saul. Without the protection of the LORD God, Saul was vulnerable to succumbing under fear. Also, this was the time of Judges (1 Samuel 1:3) during which people did whatever they saw fit and they showed no honor to God. The Spirit of the LORD was absent in the top national leader, Saul, and in Israel as a nation had little fear of God. In the absence of the fear of the LORD God, something would fill that void. That would be the fear of men. Saul and the Israel army allowed the fear of men, in this case, Goliath, to fill the void created by the absence of the fear of God.
The secondary reason for fear was the wrong focus. Note that the author intentionally described the details of Goliath’s armor, helmet, shield, and spear to highlight the splendor of them (17:4-7). Their combined weight was estimated to be more than 60lb. Saul and the Israel army allowed their vision to be captivated by the splendid armor and weapons and their ears to be captured by the taunts of Goliath. The fear that started with Saul spread to the rest of the Israel army.
Solidification of Fear: Goliath’s Eighty “Psalms”
Although the fear was contagious, it could have been contained early. However, Saul and Israel allowed the fear to solidify.
The Israel army allowed and yielded an open forum, the valley of Elah, to Goliath for the opening of his insults and mockery of the LORD God and the Israelites army freely. For the next forty days, in the morning and evening, the Philistine would come out to taunt the living God and defy the Israelites. Given that he did this twice a day, his taunt went on for eighty times, which continued to assault the mentality and spirits of the Israel army. As sound travels upward, his morning and evening taunt traveled upward and transmitted to both the Philistines and the Israelites: a booster effect on the Philistines and debilitating effect on the Israelites.
By listening to the eighty “psalms” of Goliath, the fear of Saul and the Israel army was cemented day by day, to a point of no recovery: they fled from Goliath and were greatly afraid (17:24). Because they allowed the message of the Philistine to form and cement their fear, Israel had no chance.
Fear Repelled: Identity in the Covenant of the Lord God
While Saul and the entire Israel were under fear, David came to the battlefield unaffected by their fear. When he heard the 81st “psalm “of Goliath, he responded very differently from Saul and all Israel.
Unlike Saul, David showed himself to have a firm identity in the LORD and under His covenant. He dismissed Goliath as merely someone who was uncircumcised (17:26). To David, the enormous and splendid armor and weapons were invisible, and instead he focused this battle from the perspective of God’s covenant. Then, David possibly spotted the major weaknesses of Goliath, his mobility and his large surface area. Given that he was a very tall man, his shield bearer was not able to cover his face, which David might have seen as an opening for his slingshot. With this focus, instead of allowing external visual and hearing inputs to dictate his perspective, David stayed firm within God’s truth to set his thought and perspective straight against Goliath.
What to do with fear?
Fear is powerful. However, God is far more powerful. Almost all Israel succumbed to fear when Goliath taunted and defied them. However, David had his identity firmly rooted in the LORD God to assess the overall situation from God’s perspective. He simply rejected it.
Do not let fear dictate your thoughts. Do not let fear take the open forum. Instead, squash it with your identity in the LORD God as David did.