Catechism of the Catholic Church §668 — excerpt
"Christ died and lived again, that he might be Lord both of the dead and of the living." Christ's Ascension into heaven signifies his participation, in his humanity, in God's power and authority.
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.