The priest Hawryil Kostelnyk (1886–1948), being a priest of the Ukrainian Greek-catholic Church, agreed to cooperate with the Security Services (KGB) in the Soviet Union during the period of the intensive war with the Church announced by the communist leaders. Before that he had a great authority in that Church, as a well-known philosopher, journalist and philologist. His collaboration with the authorities of the NKVD/KGB began in 1944, when visiting Moscow he decided voluntarily to initiate a close cooperation with the Soviet Security Services. So, he received from KGB an urgent and important task to fulfill: to prepare and to realize in 1946 the Council within the Greek-catholic Church (KGK) in L’viv. As the consequence of the decisions of this „Council” the KGK should to break the unity with the Vatican and start to legalize the process of the connection to the body of the Russian Orthodox Church (RKP). The main reasons of that cooperation with the KGB authorities of Fr. H. Kostelnyk still raise many doubts, as well as his death: he was killed in 1948, probably shot by a KGB officer. That is why the individual of Kostelnyk, including the details of his death were covered with many myths. His life story was kept as a secret during the period of the communist regime in Ukraine. When the communism in Ukraine has fallen, many historical researchers began to deal with the historical person of H. Kostelnyk, proving facts and declassifying the myths about his life and also about his mysterious fate. This became possible thanks to the many sources in the state archives of Ukraine which now are open for the scientific research.