Catechism of the Catholic Church §2802 — excerpt
"Who art in heaven" does not refer to a place but to God's majesty and his presence in the hearts of the just. Heaven, the Father's house, is the true homeland toward which we are heading and to which, already, we belong.
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.