The resurrection and the deposit of faith. Jesus affirms that resurrection is the work of the power of God. Paul makes himself an echo of Jesus' words. The doctrine of the resurrection is based in the Old Testament.
Christian faith is based in the resurrection.
Christian faith is based in the resurrection of Christ to the point that, if it is denied, the Christian faith would be denied completely. There is no reason to preach Jesus Christ if He hadn't risen. The gospel is based on the resurrection. Resurrection of Christ and resurrection of the believer. What Paul is teaching here is that there is such a tight union between Christ and the believer, that, if Christ did rise, the believer will also rise.