Religious individuals and communities have been at the heart of civil society and played a crucial role in the social and historical sphere of twentieth-century Korea. In particular, the Catholic Church in Korea had been widely credited for its dedication to justice for the weak and to democracy. However, it is undeniable that the Catholic Church in South Korea has lost its social influence. Indeed, over the past decade there has been a significant drop in the number of Catholics and the Church, once a pillar of civil society, has continuously lost its social position. While there are various possible explanations for this circumstance, a satisfactory one can be found in its recent past history. During the 1970s and 1980s the Church was the symbol of social and political resistance, and there was a lay leader and activist, who played a significant background role. Admittedly, Jang Ilsoon (1924-1994) is a little-known figure and thinker within Christian communities in contemporary Korea, but his teachings are far more influential among non-believers than Catholics regardless of their faith and political stance. The rationale is that he has been known to be a social activist or thinker rather than a Catholic lay leader. This is the first study to identify him as a Catholic activist and religious thinker. It aims to make an original contribution to growing interest in him and his ideological contributions to modern Korean. To scrutinise his socio-religious thought and life, this study grapples with his biographical facts and ideas, focusing on his interaction with the Catholic Church in twentieth-century Korea. As an introduction to his religious thought, this study focuses its religious background to explain how his thinking is shaped by three distinct religious ideas: Donghak, Seon Buddhism and Catholic teachings, and examines the influence of these religious ideas to grasp his thought and to understand his socio-political action. This study also discusses the way in which his religious idea can contribute to the recent pastoral realities of the Church.