In 1817 and 1818, he was a Freemason, a member and a ritual-keeper of the "Three Virtues" Lodge. He was one of the founders of pre-Decembrist secret societies Union of Salvation and Union of Welfare, and a director of the Decembrist Southern Society. He was the coordinator of correspondence between the Southern Society and the pro-Polish Society of United Slavs. Muravyov-Apostol was the author of the Decembrists "Сatechesis" and a very articulate supporter of establishing a republic and abolishing serfdom in Russia.