Covert forms of authoritarian control remain an understudied strategy of authoritarian survival. This article uses the infiltration of the Catholic Church with secret collaborators in communist Poland to study the drivers and consequences of such covert forms of control. We theorize that subnational variation in communist infiltration is driven by differences in organizational vulnerability following WWII. In turn, we argue that the uneven degree of infiltration with pro-regime agents shaped the subsequent effectiveness of the Church to foster anti-communist attitudes. We test these predictions against competing explanations (including imperial legacies and modernization) by analyzing seven Polish surveys from the late communist period (1985-89). Our results confirm the importance of organizational vulnerability in driving the success of communist infiltration efforts and suggest that infiltration with secret agents was effective in undermining the Church’s ability to shape the political attitudes of frequent churchgoers.