His apostolic zeal became even greater as a bishop when he tended to abandoned people in the peripheries and encouraged the work of a range of different religious orders.
Rosaz began to serve as a chaplain in prisons and he also opened a retreat for girls in 1862 while later befriending Giovanni Bosco; this friendship later saw him appointed as the rector of seminarians in Susa from 1874 to 1877.
He wept upon learning he was to be made a bishop and sent a letter to Cardinal Giacomo Antonelli protesting the appointment in spite of his lack of theological or canon law doctorates.
He encouraged religious orders (such as the Conventual Franciscans and the Salesians) in his diocese and founded a diocesan newspaper in 1897 titled "Il Rocciamelone".
[1][3] Rosaz made a pilgrimage to Novalesa on 13 March 1862 and in 1872 travelled to Savona to collaborate with Maria Giuseppa Rossello in the foundations of her new religious order.