Subbotniks (Russian: Субботники, IPA: [sʊˈbotnʲɪkʲɪ], "Sabbatarians") is a common name for adherents of Russian religious movements that split from Sabbatarian sects in the late 18th century.
The majority of Subbotniks were converts to Rabbinic or Karaite Judaism from Christianity. Other groups included Judaizing Christians and Spiritual Christians.
Conversion to Judaism (Hebrew: גִּיּוּר, romanized: giyur or Hebrew: גֵּרוּת, romanized: gerut) is the process by which non-Jews adopt the Jewish religion and become members of the Jewish ethnoreligious community. It thus resembles both conversion to other religions and naturalization. The procedure and requirements for conversion depend on the sponsoring denomination. Furthermore, a conversion done in accordance with one Jewish denomination is not a guarantee of recognition by another denomination. Normally, though not always, the conversions performed by more stringent denominations are recognized by less stringent ones, but not the other way around. A formal conversion is also sometimes undertaken by individuals who are raised Jewish or have Jewish ancestry but who may not be considered Jewish according to stringent interpretations of traditional Jewish law.
There are some groups that have adopted Jewish customs and practices. For example, in Russia the Subbotniks have adopted most aspects of Judaism without formal conversion to Judaism. However, if Subbotniks, or anyone without a formal conversion, wish to marry into a traditional Jewish community or immigrate to Israel under the Law of Return, they must have a formal conversion.