The doctoral thesis tries to reflect the collaboration of Catholic priest with the State Security Service (ŠtB). This is done in two steps. The first part illustrates the historical background based on research of archive materials and testimony of living priests - agents. Results of historical research tell us that it is not possible to assess the collaboration across the board just on the registration of agents in secret police register. Core of historical part is a typology of collaboration. It serves as a base for input to the second ethical part of doctoral thesis. The main goal of ethical part is to find objective criterions for answerable and ethical way out from moral dilemmas. Core of this part is theory about ethical compromise with two normative principles - principle of small evil and principle of double effect. The analysis of small evil didn't get a satisfy way out from the situation of blackmail: in the concrete situation it didn't tell us, which evil is smaller and which is bigger. The principle of double effect showed us, that the collaboration in blackmail situation is without proportionate cause, in other words, is counterproductive to their value in universal perspective. The priests hadn't any guaranty, that after consent with collaboration in blackmail, the damage on their side but...