The Decree against Communism is a 1949 Catholic Church document (by Pope Pius XII) which excommunicates all Catholics collaborating in communist organizations. The document resulted in one of the largest formal excommunications in the history of the Catholic Church (it could include more than several million Catholics).
The Vatican, having been silent during the war on communist excesses, displayed a harder line on communism after 1945. The ruling followed suit to an earlier 1937 encyclical entitled Divini Redemptoris which was strongly critical of communism and its Christian variants.
The Holy Office issued several decrees, falling broadly into two categories:
On July 15, 1948, L’Osservatore Romano published a decree about communism, which excommunicated those who propagate "the materialistic and anti-Christian teachings of communism", which was widely interpreted as an excommunication of the Communist Party of Italy, which however, was not mentioned in the decree.[1] The Sanctum Officium continued to issue condemnations:
The decree was confirmed in 1962 by Pope John XXIII when it was announced that Fidel Castro would be excommunicated for embracing Communism and persecuting members of the Catholic Church.