Catechism of the Catholic Church §644 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.