Believing things to be true when we don't and can't actually know to be so is generally not a good idea. It's basically a form of self-deception that tends to separate us from reality. And therefor from truth.
On the other hand, choosing to trust in a truth that is possible, even though unknowable, and then acting as if, can often result in bringing about the truth that we were hoping for. And that's called acting on faith, as opposed to belief. The key difference being that faith does not ignore the possibility that the truth we hope for may or may not manifest when we act on it. That keeps us grounded in reality while allowing us to act on our hope.