Catechism of the Catholic Church §2613 — excerpt
Three principal parables on prayer are transmitted to us by St. Luke: - the first, "the importunate friend," invites us to urgent prayer: "Knock, and it will be opened to you."
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.