What does 1 Samuel 4:8 mean?

From the perspective of Israel's ancient Near East neighbors, the "gods" of the Israelites had a reputation for demonstrating real power. Though the Philistines did not understand that the Israelite "gods" were actually a single deity—the One True God of all things—they did know that the Lord struck the Egyptians with every sort of plague when He was freeing His people from their slavery to Egypt. The story of Israel's exodus following the plagues in Egypt had not faded from the memory of the surrounding nations. They still feared the Lord of Israel even years later.

When they hear that the ark of God is in Israel's camp, the Philistines are terrified. Because of their wrong view of idols, they are certain the ark of the covenant is God Himself. Woe—or "alas"—to us! the Philistines cry out. They do not trust their own god Dagon (1 Samuel 5:2–5) to deliver them from the power of the God of Israel.

The theology of the Israelites in their own camp is not much better. God's presence sometimes came to rest on the ark of the covenant, in a narrow sense (Leviticus 16:2). Israel is misinterpreting this to mean God's power can be wielded like a mindless tool or weapon. They think He is obliged to give them victory over the Philistines simply because the ark is near the battlefield. They have failed, though, to repent of their own worship of false gods and to ask the Lord to help them. Samuel will call them to do so twenty years later, leading to much different results (1 Samuel 7).