In the earliest days of Christianity an ‘apostle’ was first and foremost a man who claimed to be an eyewitness of the Resurrection. Only a few days after the Crucifixion when two candidates were nominated for the vacancy created by the treachery of Judas, their qualification was that they had known Jesus personally both before and after His death and could offer first-hand evidence of the Resurrection in addressing the outer world (Acts). A few days later St Peter, preaching the first Christian sermon, makes the same claim—‘God raised Jesus, of which we all (we Christians) are witnesses’ (Acts ). In the first Letter to the Corinthians, St Paul bases his claim to apostleship on the same ground—‘Am I not an apostle? Have I not seen the Lord Jesus?’ (1:9).
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 Christians 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 ever have been written.
From Miracles by C S Lewis