I thought Hungary under Bela Kun or Mongolia in the 1920s might have that distinction. Albania's turning away from God is good ammo to use against secularists.
The Albanian Church triumphed — at a high price. Of 156 priests before the persecution began, 65 were martyred, 64 died during or after imprisonment. Tens of thousands of common people perished for religious reasons. No people passed through a worse trial in the 20th century.
Pope John Paul II has said: “History has never seen before what happened in Albania. Dear Albanians, your drama must interest the whole European continent: Europe must not forget.”