Huh? Did you read what you wrote here? CMR only proves there was some sort of a beginning, a BIG BANG. It doesn’t provide many clues to what the shape of the Universe might be. There are different mathematical models to explain possible shapes of the universe. We don’t really know but there are many theories. One of which is probably right but we can’t be sure unless we could observe it and maybe somehow test it. For us, finite may be as good as infinite since we can’t observe or test it yet. No, it’s not a belief. It’s called Science. It evolves, grows, and improves with time and brilliant minds.