This thesis examines the emergence and influence of liberal theology within the Transylvanian Reformed Church through the life and work of Bishop Péter Nagy. The aim of the research is to explore how nineteenth-century theological rationalism and liberalism appeared in Transylvania, what its principal teachings were, and how it influenced Reformed theological thought and religious education. The first chapter presents the origins and main characteristics of liberal theology, with special attention to its understanding of revelation, Scripture, and the person of Jesus Christ. This is followed by an overview of the historical background of its spread in Hungary and Transylvania. The third chapter discusses the life and ecclesiastical activity of Péter Nagy, while the final chapter analyzes his smaller and larger catechisms. Based on the results of the study, it can be concluded that several characteristics of liberal theology appear in Péter Nagy's work and catechisms, reflecting a significant shift in theological perspective in comparison with reformation theology.