Congrégation de l'Oratoire
Giustiniani was born the Island of Chios, then part of the Republic of Genoa, to the powerful Giustiniani family. He was a relative of two Bishops of Chio - Girolamo Giustiniani (1599–1604) and Marco Giustiniani (1604–1640). He entered the Congregation of the Oratory of Saint Philip Neri in 1603, only 8 years after the death of Philip Neri.