Sodalitium Pianum is Latin for "the fellowship of Pius" (referring to Pope Pius X or Pius V). It can refer to two different groups.
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In Roman Catholicism
In reaction to the movement within the Roman Catholic Church known as Modernism, Pope Pius X issued in 1907 the encyclical Pascendi dominici gregis and the decree Lamentabili sane exitu, which condemned the movement as a heresy.
To ensure enforcement of these decisions, Monsignor Umberto Benigni organized, through his personal contacts with theologians, an unofficial group of censors who would report to him those thought to be teaching condemned doctrine. This group was called the Sodalitium Pianum, i.e. Fellowship of Pius (X), which in France was known as La Sapinière. It never had more than fifty members, but its frequently overzealous and clandestine methods, including opening and photographing private letters, and checking out the records of the local bookshop to see who was buying what, hindered rather than helped the Church's combat against Modernism.
Cardinal Secretary of State Rafael Merry del Val prevented the association from gaining canonical recognition, and the competent department of the Roman Curia disbanded it in 1921 on the grounds of "changed circumstances". According to Yves Congar O.P., the network remained operational to some degree until the early years of the Second World War.
Researchers are divided in their opinions about the extent to which Pius X was aware of or approved Benigni's initiatives.
In sedeprivationism
In 1985, the name Sodalitium Pianum was adopted by a sedeprivationist group of Traditionalist Catholics also known as Istituto Mater Boni Consilii, an offshoot of the Traditionalist Society of St. Pius X.