descristianizar

To de-Christianize — to remove or suppress Christian religious influence from a society or institution. It's the verb behind descristianización, the historical term for the French Revolution's 1793–94 campaign against the Catholic Church, which abolished Christian worship and briefly installed the state cult of the Goddess Reason.

• La Revolución francesa buscó descristianizar la sociedad. — The French Revolution sought to de-Christianize society.
• El calendario revolucionario fue parte del intento de descristianizar Francia. — The revolutionary calendar was part of the attempt to de-Christianize France.
• Historiadores debaten cuán profundamente logró descristianizar al país. — Historians debate how deeply it managed to de-Christianize the country.

It's a historical/academic term almost always tied to specific episodes like the French Revolution — using it casually for someone simply leaving the Church would sound oddly grandiose; dejar de practicar or alejarse de la religión fits everyday contexts better.

• politics Descristianización de Francia durante la Revolución — The 1793–94 campaign by French revolutionary factions, intensified during the Terror, that abolished Christian worship, persecuted clergy, and briefly replaced Catholicism with the state-sponsored Cult of Reason — a standard reference point in histories of the French Revolution.