This work seeks to determine the origins of preaching in Christianity, and to trace its history before Origen. On the basis of a examination of the external evidence for Christian preaching before Origen and of cognate activities in the ancient world which might have influenced Christian practice, and on the basis of a narrative hypothesis on the nature of the development of Christianity, a history is traced by which prophecy gives way to Scripture as the primitive Christian oikos becomes the oikos theou. The homily is seen to emerge from the practice of submitting prophecy to judgement and application, which comes to employ Scripture and in time is employed on Scripture itself. This is an attempt to answer the questions of how, when and why preaching entered Christian worship.