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. Ethnic assimilation and loss of German language have been forced by the deportation of 1941 and special settlement. The number of Mennonite identity carriers significantly dropped due to the bitter struggle with religion. Many Mennonites joined the Evangelical Christian-Baptists producing a further erosion of identity. After decline of religious persecutions, reproduction of Mennonite identity could take place in rare Mennonite congregations and families. As a result, Mennonite identity did not disappear but significantly changed.