Gemistus Pletho
The Byzantine philosopher Gemistus Pletho (c. 1355/1360 – 1452/1454) secretly advocated polytheism in his book "Nomoi", which he only circulated among close friends. After his death it came into the hands of the Ecumenical Patriarch of Constantinople Gennadius II, who eventually burned it, although Gennadius' summary of its content survives in a letter.