Hendrik Kraemer (18881965) was one of the outstanding Christian thinkers and activists in the 20th century. He studied linguistics and relgions in University of Leiden, Netherlands, and Islam and Arabic in University of Al Azhar, Egypt. After acquiring those academic training, he went to Indonesia to help translating Alkitab, the Indonesian Bible. In 1937 Kraemer came back to Netherlands and became a professor of comparative religion in University of Leiden. After spending 10 years in there, he was appointed as the first director of the Ecumenical Institute in Bossey near Geneva. In his latter years (19561957) he was invited from Union Theological Seminary in New York as a guest lecture. The most significant contribution of Kraemer is to define the meaning of revelation of God in relation to other religions. He insisted that God is only and perfectly revealed himself in Jesus Christ according to the Bible. Any other religion or philosophy cannot influence to the revelation of God at all. This is the basic concept of Biblical Realism. Christianity is clearly distinguished from other religions, but simultaneously Kraemer warned a superiority of Christianity over them. Kraemer claimed that the nature of church is both mission and diakonia. In and through missionary activity church reveals her reason for existence. Church should be “WorldCentered” rather than “Being ChurchCentered.” If she mainly emphasizes her own organization instead of ‘World,’ it is a betrayal of calling. In that sense, the role of lay people is crucial. According to Kraemer, the definition of lay people is different. They are not a group of people who contrast to ordained pastors, but a chosen people of God. As a lay person, Kraemer paid attention to roles of them in relation to mission in Islamic context. They should participate in the mission of God in everyday and everyplace.