Acts 1:21-23 it states the purpose of an apostle: to be a witness of the resurrection. Paul is able to base his calling of apostleship in 1 Cor 9 on his vision of the resurrected Jesus on the road to Damascus (Acts 9).
The message was entrusted first to the Apostles, of whom Paul counted himself as one. The cornerstone of the message was this: Jesus Christ was raised from the dead. Everything else flows from this one fact. If Jesus did not rise from the dead then our faith is futile.