Who were non-Christians in the Roman Empire?

Most people who lived in the Roman Empire were non-Christians until the Empire itself became Christian (and even then, there were many non-Christians). Most people who lived in the Roman Empire were polytheists (especially the Romans; remember, the Romans conquered many other peoples), though not all believed in the same gods. Others were Jews or had other religious beliefs. There were basically no atheists.

Another answerNon-Christians were called pagans.