The world does not consist of 100 percent Christians and 100 percent non-Christians. There are people (a great many of them) who are slowly ceasing to be Christians but who still call themselves by that name: some of them are clergymen. There are other people who are slowly becoming Christians though they do not yet call themselves so.
A missionary church cannot rely on the professional ministry for the primary work of mission. The role of the laity is critical because it is the lay members of the church who have the greatest contact with those who are outside of the normal structures of church life. In such a situation the task of clergy is not so much to engage in mission themselves, as to support the laity in their mission.