Catechism of the Catholic Church §2510 — excerpt
The golden rule helps one discern, in concrete situations, whether or not it would be appropriate to reveal the truth to someone who asks for it.
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.