If one can only be saved by Catholicism, this means that all non-Catholic Christians are unbelievers. Correct?

No, this is not according to Catholic teaching at all, and you obviously do not understand the Catholic faith at all. If you did, you could never have left it for anything less. The Catholic Church does NOT teach that non-Catholics, or non-Christians for that matter, cannot be saved. That is a teaching of fundamentalist Protestantism - its their church or hell.