The Garden Tomb
"In the earliest days of Christianity an 'apostle' was first and foremost a man who claimed to be an eye-witness 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 i. 22). 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 ii. 32). 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?'"
C. S. Lewis, Miracles, London: Geoffrey Bles, 1947, p. 171.