Catechism of the Catholic Church §1663 — excerpt
Since marriage establishes the couple in a public state of life in the Church, it is fitting that its celebration be public, in the framework of a liturgical celebration, before the priest (or a witness authorized by the Church), the witnesses, and the assembly of the faithful.
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.