When we talk about “beliefs” and “believing in” things, we’re usually talking about stuff we don’t understand or can’t explain: We believe in ghosts and gods. We believe everything happens for a reason.
We don’t believe in trees, though. We know a lot about trees.
But to believe also means to have strong feelings about something, like how people should treat each other or what kinds of things would make the world a nicer place.
It can be confusing when one word has two different meanings. If you say you don’t believe in a god, sometimes people will look at you suspiciously and say, “But don’t you believe in love?”
Do you see what’s going on here? The (wrong) idea is that if you don’t do the kind of believing where you think there’s a supreme being watching over you, you probably also don’t have strong feelings about things that are hard to quantify, like love or compassion or joy.
Like I said, it can be confusing.