Sunday of the Resurrection

The faith of the first community of believers [in the Resurrection of the Jesus Christ] is based on the witness of concrete men known to the Christians and for the most part still living among them. Peter and the Twelve are the primary "witnesses to his Resurrection", but they are not the only ones - Paul speaks clearly of more than five hundred persons to whom Jesus appeared on a single occasion and also of James and of all the apostles (1 Cor 15:4-8; cf. Acts 1:22.).
Given all these testimonies, Christ's Resurrection cannot be interpreted as something outside the physical order, and it is impossible not to acknowledge it as an historical fact. It is clear from the facts that the disciples' faith was drastically put to the test by their master's Passion and death on the cross, which he had foretold (cf. Lk 22:31-32). The shock provoked by the Passion was so great that at least some of the disciples did not at once believe in the news of the Resurrection. Far from showing us a community seized by a mystical exaltation, the Gospels present us with disciples demoralized ("looking sad”, (Lk 24:17; cf. Jn 20:19) and frightened. For they had not believed the holy women returning from the tomb and had regarded their words as an "idle tale" (Lk 24:11; cf. Mk 16:11,13). When Jesus reveals himself to the Eleven on Easter evening, "he upbraided them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen” (Mk 16:14).
Even when faced with the reality of the risen Jesus the disciples are still doubtful, so impossible did the thing seem: they thought they were seeing a ghost. "In their joy they were still disbelieving and still wondering” (Lk 24:38-41). Thomas will also experience the test of doubt and St. Matthew relates that during the risen Lord's last appearance in Galilee "some doubted” (Cf. Jn 20:24-27; Mt 28:17). Therefore, the hypothesis that the Resurrection was produced by the apostles' faith (or credulity) will not hold up. On the contrary their faith in the Resurrection was born, under the action of divine grace, from their direct experience of the reality of the risen Jesus.
 
Catechism of the Catholic Church, paras. 642 - 644