We find in Chrétiens sans Église a massive amount of information to study the history of secularization (or the process of disaffiliation from denominations) at the beginning of the modern era, while the author claims to rule out of his study the movements leading to the end of religion. This excessive investigation in comparison with the enunciated project is symptomatic of a series of theoretical and methodological instabilities: a disciplinary instability – between philosophy and history –; method instabilities between the construction of ideal models and empirical historiographic work, between structural interpretation and genetic explanation; a philosophical instability between historic materialism and existential phenomenology. We then can assume that these instabilities reveal a dual approach with regard to Communist ideology, made up of both adhesion and dissent. Chrétiens sans Église, that studies the issue of revolutionary movements’ betrayal within their institutionalization could then be read as a kind of veiled archeology of the Christian roots of Communism.