déchristianiser

To dechristianize — to strip Christian symbols, practices, or influence from public or social life. Historically tied to the French Revolution's violent déchristianisation campaign of 1793-1794 (closing churches, renaming streets, replacing the calendar), and used today in sociology to describe France's long-term decline in religious practice.

• La Convention a cherché à déchristianiser la France pendant la Terreur. — The Convention sought to dechristianize France during the Terror.
• Les sociologues étudient comment la société française s'est progressivement déchristianisée. — Sociologists study how French society gradually became dechristianized.
• Le nouveau calendrier révolutionnaire visait à déchristianiser le temps lui-même. — The new revolutionary calendar aimed to dechristianize time itself.

A historical and sociological term about institutions or whole societies, not about an individual person's private beliefs — you wouldn't say a single person "se déchristianise" the way you'd say they lose their faith.