Throughout his writings on social, political, and cultural philosophy--in short, public philosophy--Catholic philosopher Jacques Maritain (1882-1973) rejects a sharp division within Christian life between nature and grace. This rejection has important implications for his view of the human person, the kingdom of God, the Church, and the nature of Christian involvement in the world. My thesis in this article is that the debate over Christian witness in public life today can, in some ways, be seen as a dispute over a proper theological and philosophical understanding of the relationship between nature and grace. In section 1, Maritain's integral humanism is presented where man is considered in the totality of his natural and supernatural being. Section 2 offers an extended analysis of the theology of nature and grace that underpins his integral humanism. This theology has implications for Maritain's view of Christian witness in a pluralistic society, as well as the relationship between the state, the Church, and the kingdom of God. These implications are examined in sections 3 and 4, respectively. I conclude in section 5 with a summary of the fundamental ideas of Maritain's public philosophy.