Acto - Ailb
- Acton, Charles Januarius
- Acton, John
- Acton, John Emerich Edward Dalberg, Baron Acton
- Acton, John Francis Edward
- Acts of Roman Congregations
- Acts of the Apostles
- Acts of the Martyrs
- Acts, Canonical
- Acts, Human
- Acts, Indifferent
- Actual Grace
- Actus et Potentia
- Actus primus
- Actus Purus
- Acuas
- Ad Apostolicae Dignitatis Apicem
- Ad Limina Apostolorum
- Ad Limina Visit
- Ad Sanctam Beati Petri Sedem
- Ad Universalis Ecclesiae
- Adalard, Saint
- Adalbert
- Adalbert I
- Adalbert Stifter
- Adalbert, Saint
- Adam
- Adam Blackwood
- Adam Contzen
- Adam de Marisco
- Adam Easton
- Adam Franz Lennig
- Adam Heinrich Mueller
- Adam in Early Christian Liturgy and Literature
- Adam Krafft
- Adam Mickiewicz
- Adam of Bremen
- Adam of Fulda
- Adam of Murimuth
- Adam of Perseigne
- Adam of St. Victor