As this qualification suggests, to preach Christianity meant primarily to preach the Resurrection. .... The Resurrection is the central theme in every Christian sermon reported in the Acts. The Resurrection, and its consequences, were the "gospel" or good news which the Christian brought: what we call the "gospels", the narratives of Our Lord's life and death were composed later for the benefit of those who had already accepted the gospel. They were in no sense the basis of Christianity: they were written for those already converted. The miracle of the Resurrection, and the theology of that miracle, comes first: the biography comes later as a comment on it. ....The first fact in the history of Christendom is a number of people who say they have seen the Resurrection. If they had died without making anyone else believe this 'gospel' no gospels would have ever been written.
C.S. Lewis, Miracles