I
- Ian Theodor Beelen
- Ibague
- Ibarra
- Ibas
- Ibora
- Iceland
- Iconium
- Iconoclasm
- Iconostasis
- Ida Hahn-Hahn
- Idaho
- Idea
- Idealism
- Idiota
- Idolatry
- Idumea
- Iglesias
- Ignace Bourget
- Ignace-Gaston Pardies
- Ignacio Barbosa-Machado
- Ignacio de Arbieto
- Ignacio de Iriarte
- Ignatius Carbonnelle
- Ignatius Knoblecher
- Ignatius Krasicki
- Ignatius Mrak
- Ignatius Persico
- Ignatius von Weitenauer
- Ignaz Assmayer
- Ignaz Heinrich von Wessenberg
- Ignaz Kogler
- Ignaz Philipp Semmelweis
- Ignazio Danti
- Ignorance
- IHS Monogram
- Il Parmigiano
- Il Tintoretto
- Illegitimacy
- Illinois
- Illinois Indians
- Illuminated Manuscripts
- Illuminati
- Illuminati
- Illyria
- Imagination
- Imitation of Christ
- Immaculate Conception
- Immanence
- Immortality
- Immunity
- Imola
- Impanation
- Impediment of Crime
- Imposition of Hands
- Imposters
- Improperia
- In Coena Domini
- In Commendam
- In Partibus Infidelium
- In Petto
- Incardination and Excardination
- Incense
- Incest
- Index of Prohibited Books
- India
- Indiana
- Indianapolis
- Indifferent Acts
- Individual, Individuality
- Individualism
- Indo-China
- Induction
- Indulgences
- Industrial Conciliation
- Infallibility
- Infamy
- Infanticide
- Infidels
- Infinity
- Influence of the Church on Civil Law
- Infralapsarians
- Ingulf
- Inigo Jones
- Injustice
- Innocenzo di Pietro Francucci da Imola
- Inquisition
- Insanity
- Inscription of Abercius
- Inspiration of the Bible
- Installation
- Instinct
- Institute of Mary
- Institute of Mission Helpers of the Sacred Heart
- Institute of Notre-Dame de Namur
- Institute of the Brigidines
- Institute of the Divine Compassion
- Institute of the Divine Compassion
- Institute of the Divine Compassion
- Institutes of Bon Secours
- Intellect
- Intendencia Oriental y Llanos de San Martin
- Intention
- Intercession
- Interdict
- Interest
- Interims
- International Law
- Internuncio
- Interpretation of Dreams
- Introit
- Intrusion
- Intuition
- Inventory of Church Property
- Invitatorium
- Ionian Islands
- Ionian School of Philosophy
- Ionopolis
- Iowa
- Ippolito Pindemonte
- Ipsus
- Ireland
- Irenopolis
- Irish College, in Rome
- Irish Colleges on the Continent
- Irish Confessors and Martyrs
- Irish Institute of the Blessed Virgin Mary
- Irish Literature
- Irnerius
- Iroquois
- Irregularity
- Irremovability
- Irvingites
- Isaac
- Isaac of Armenia
- Isaac of Nineveh
- Isaac of Seleucia
- Isaac Thomas Hecker
- Isaac-Joseph Berruyer
- Isabella I
- Isaias
- Isaura
- Ischia
- Isernia and Venafro
- Isidore of Thessalonica
- Isionda
- Islam (Concept)
- Isleta Pueblo
- Ismael (Ishmael)
- Ispahan
- Israel
- Issac Austin Henderson
- Issachar
- Issus
- Italian Literature
- Italians in the United States
- Italo-Greeks
- Italy
- Ite Missa Est
- Itineraria
- Itinerarium
- Ivan Sergejewitch Gargarin
- Ivory
- Ivrea
- St. Ibar
- St. Ignatius Loyola
- St. Ignatius of Antioch
- St. Ignatius of Constantinople
- St. Ildephonsus
- St. Illtyd
- St. Ine
- St. Irenaeus
- St. Isaac Jogues
- St. Isabel of France
- St. Isidore of Pelusium
- St. Isidore of Seville
- St. Isidore the Labourer
- St. Ita
- St. Ives (Yves)
- The Incarnation
- The Irish (In Countries Other Than Ireland)