Why is the captain's "small idea to believe in"?

The captain at least wanted to keep the bodies of his men together,

“I took my orders, too. But if i couldn't keep you alive, I thought I could at least keep you together. In the middle of a big war, you go looking for a small idea to believe in. When you find one, you hold it the way a soldier holds his crucifix when he's praying in a foxhole.”