Members of a sect who, though not Jews, hold to the keeping of the Jewish Sabbath, rather than the Christian Lord's Day.
Some of them, while observing Sunday, have sought to make it as rigorous as the Sabbath of the Old Law; others, more consistently, keep the seventh day of the week instead of the first, as a day of rest.
The former have been found in various Protestant bodies, the latter are generally members of such sects as the Adventists, Seventh-Day Baptists, etc.
After the settlement of the Judaising controversies of the 1st century, Sabbatarianism arose again in the 16th century, and has continued until the present time.
New Catholic Dictionary