Kant maintained that what we know about the world around us can only be about the way the world appears to us, not the world as it is in itself. Therefore, we are left with just putting our appearances together, and given this we can't expect there to be evidence either way. So, Kant said, we have to decide whether to believe in God or not depending on whether we think it would make us a better person if we believed. And, he thinks that belief in God would be better for our character, and therefore we should believe.
Contrast him with someone like Richard Dawkins, of God Delusion fame. Dawkins thinks that if there were a God, there would be evidence for his existence. But, there isn't any evidence that isn't better explained by evolution. He thinks we can know reality, and it makes more sense without belief in God. In fact, he actually thinks God couldn't possibly explain anything at all, and it would be the ultimate wrong answer whether or not there was a good evolutionary explanation for everything. For him, absence of evidence is not evidence of absence. To believe in God is to embrace a belief that is almost certainly false, and therefore cannot be moral.