'Unbeliever' in the Bible

But if all prophesy [foretelling the future, speaking a new message from God to the people], and an unbeliever or outsider comes in, he is convicted [of his sins] by all, and he is called to account by all [because he can understand what is being said];

What harmony can there be between Christ and Belial (Satan)? Or what does a believer have in common with an unbeliever?

If anyone fails to provide for his own, and especially for those of his own family, he has denied the faith [by disregarding its precepts] and is worse than an unbeliever [who fulfills his obligation in these matters].