"Fear not, stand firm, and see the salvation of the LORD” (v13). The Israelites lost their courage when they saw that they were trapped between the Red Sea and the Egyptian army. But God wanted them to recognize His power over the Egyptians and His ability to save His people. God required nothing from the Israelites but trust: “The Lord will fight for you, and you have only to be silent” (v14).
“So the people feared the LORD, and they believed in the LORD” (v31). We “fear” God (and no one else) because He is the Creator and ultimate Judge; we trust Him because He loves us and has promised to fight for us.