When Eusebius set out to write an Ecclesiastical History he claimed to be ‘the first to undertake this present project and to attempt, as it were, to travel along a lonely and untrodden path’. The claim was justified: there had been little room for religious history, even the history of pagan religions, in the works of classical historians and their imitators. Following the rules laid down by Thucydides, they concentrated on the political life of the present and its military consequences; they preferred oral to written sources, provided the historian had either been present at the scene of action or had heard reports from eyewitnesses. Both in method and in content Eusebius was an innovator. Since his starting point was ‘the beginning of the dispensation of Jesus’ he was entirely dependent on written sources for more than three hundred years; and, innovating still more, he introduced documents such as letters and imperial edicts into his narrative. Far from being political and military, his subject matter was primarily the history of the apostles, the succession of bishops, the persecutions of Christians, and the views of heretics. He was widening the scope of historical writing and using the techniques previously employed in the biographies of philosophers. It is not surprising that, once his work had been translated into Latin and extended by Rufinus and Jerome, it became the starting point for writers on ecclesiastical history for generations to come.