 He studied at the Pontifical Roman Seminary (minor and major branches), the Pontifical Lateran University (where he obtained his doctorates in theology and canon and civil law), and the Pontifical Institute of Christian Archeology in Rome.Ordained to the priesthood on 24 February 1923, Ferretto finished his studies in 1926 and then taught at the Pontifical Lateran University and the Pontifical Urbaniana University until 1958.He served as an official in the Vicariate of Rome from 1929 to 1939, when he was made a referendary of the Apostolic Signatura on 23 April.