Catechism of the Catholic Church §1577 — excerpt
"Only a baptized man (vir) validly receives sacred ordination." The Lord Jesus chose men (viri) to form the college of the twelve apostles, and the apostles did the same when they chose collaborators to succeed them in their ministry.
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.
Whole chapters cited: Titus 44 · Titus 300
Also cites: CIC, can. 1024 · St. Clement of Rome, Ad Cor. 42, 4 · Titus 3: PG 1 · John Paul II, MD 26-27 · CDF, declaration, Inter insigniores: AAS 69 (1977) 98-116