The main aim of this thesis is to add to the understanding of how religion interacts with politics in modern Europe. It discusses the largely under-researched relationship between Euroscepticism and religion and argues that despite the theory of secularisation, which assumes an ever-increasing marginalisation of the role of religion in the public sphere, religious beliefs still play an important role in the world of politics. This study firstly analyses the phenomenon of political Catholicism, providing a concise definition and identifying the primary and secondary features of this movement. Secondly, it discusses how, under certain circumstances, political Catholicism, which in general is a movement that strongly supported European integration, can adopt a Eurosceptic stance. Empirically, this study focuses on the case of Poland, but places the results in the context of 15 other European countries. It analyses primary data gathered through documentary research (party manifestos, public statements, Church documents) and a number of interviews with party members. Additional information was collected from interviews with journalists, an expert survey and secondary literature in the field of EU studies and religion and politics. The key argument of this thesis is that Polish political Catholicism is a distinct phenomenon in Europe because of its markedly visible Euroscepticism. Its opposition to Europe stems from an intrinsic feature of the movement, namely its Catholic-nationalist nature. This attribute developed as a result of Polish history, characterised by long periods of struggle for independence from alien forces, whereby belonging to Catholicism gradually became an essential element of belonging to the Polish nation, helping to define the 'Us' vs. the 'Other'. Consequently, Polish political Catholicism started perceiving the EU as an organisation based on 'alien', secular values, which could endanger Polish national identity, Polish society and ultimately the state. In broader terms, this study contributes to the general discussion on the causality of party-based Euroscepticism. It also provides a comparative outlook on political Catholicism in Europe, arguing that there is a gap between Western and Central and Eastern European strands of this movement, resulting mainly from different historical circumstances in which the movement developed in both regions.