Catechism of the Catholic Church §185 — excerpt
Whoever says "I believe" says "I pledge myself to what we believe." Communion in faith needs a common language of faith, normative for all and uniting all in the same confession of faith.
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.