Msgr. Pope has an excellent post on when the Resurrection becomes the official teaching of the Church. The proclamation of Christ's Resurrection, of course, is part of the original kerygma (Jesus is Lord, Jesus lived and died as a real man, Jesus rose from the dead). That kernel of the faith still underlies everything the Church is and does.
Here's the heart of Msgr.'s point in this post:
So when does the resurrection become the official declaration of the early Church? Up till now the stories had been rejected by the apostles as either fanciful or untrue. Even the possible belief of one of the 12 (John) was not enough to cause an official declaration from the early Church. So, what causes this to change? It would seem that, after the early evening report by the disciples returning from Emmaus, Peter slipped away, perhaps for a walk, or some other purpose, and according to both Paul (1 Cor 15:5) and Luke (Lk 24:34) the risen Lord appeared to Peter privately and prior to the other apostles. Peter then reports this to the others, and the resurrection moves from being doubted, to being the official declaration of the community, the Church. The official declaration is worded thus:
The Lord has truly risen indeed, he has appeared to Simon!” (Luke 24:34)
The resurrection is now officially declared. Notice, the world “truly” (some texts say “indeed”). It is now an officially attested fact that Jesus has risen. Neither Magdalene, nor the women in general, nor the disciples from Emmaus, nor even John, could make this declaration for the Church. It took the college of apostles in union with Peter to do this. Hence the dogma of the resurrection becomes so on very Catholic terms: The first bishops (the apostles) in union or in Council with the first Pope (Peter) make this solemn declaration of the faith.
I heartily agree; and the whole thing is well worth reading.