The article analyses the multiple means and different uses of the term “lay” including its derivations, from the beginning of the 19th century. It then examines the cultural and religious transformations generated by the confrontation between the Christian Churches and the currents of “secular” thought on the one hand and the totalitarian regimes and ideologies of the 20th century on the other. Thirdly, on the basis of the distinction between “laity” meaning the civil sphere of the State and “secularism” in the sense of an intellectual and moral attitude, the article treats the outcome in the form of post-war constitutions and politics in western democracies. Finally the author discusses factors and circumstances that in the last few years have reopened the question and kindled the public debates on issues neglected for some decades.