Catechism of the Catholic Church §1722 — excerpt
Such beatitude surpasses the understanding and powers of man. It comes from an entirely free gift of God: whence it is called supernatural, as is the grace that disposes man to enter into the divine joy.
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.
Also cites: St. Irenaeus, Adv. haeres. 4,20,5: PG 7/1, 1034-1035