The article examines historical roots and dynamics of Mennonite identity in USSR using the concept of an ethno-confessionality, introduced to Soviet religious studies by Alexei N. Ipatov. As a result of secularization, in the beginning of the 20th century a part of Mennonites gave up religion but continued to regard themselves Mennonites by culture, and Mennonite identity ceased to be strictly religious. During the Civil War, the confessional identity lost its traditional core — obligatory nonresistance.