His family and the aristocracy looked forward to a most brilliant career as a statesman for him, but at the same time he joined the Congregation of the Holy Virgin (French: Congrégation de la Sainte-Vierge), a religious association of the laity organized in Paris in 1801, which had grown out of the Jesuit-affiliated Sodality of Our Lady after the Suppression of the Society of Jesus.
There he met a group of young seminarians at Saint-Sulpice who were caught up in the idea of missionary work, and the impetuous Forbin-Janson became imbued with this vision.
He became close friends with Eugène de Mazenod, who entertained similar dreams,[3] and who was later to found the missionary order of the Oblates of Mary Immaculate.
Later, in 1814, while serving as the acting Vicar General of the diocese, he traveled to Rome where Pope Pius VII advised him to remain in France where missionary work was needed.
[3] He immediately heeded the advice, and, with his friend, the Abbé Jean-Baptiste Rauzan, re-established the Missionaries of France, later called the Society of the Fathers of Mercy, based at Mont-Valérien, in the Parisian suburb of Suresnes.
[3] In 1817 Forbin-Janson was sent to Syria on a mission, returned to France in 1819, and again took up the work of a missionary in his homeland until 1823 when he was appointed Bishop of Nancy and Toul and Primate of Lorraine, for which he was consecrated at the chapel of the Society on Mont-Valérien on 6 June 1824 by Gustave Maximilien Juste de Croÿ-Solre, the Archbishop of Rouen.
[3] Seen as a problem by the new government of King Louis Philippe for his views and his refusal to sign the Declaration of the Clergy of France of 1682 which was rejected by the Holy See, he was not allowed to return to his diocese.
He went to Rome, where Pope Gregory XVI supported his vision and gave him an official mandate for a missionary tour through the United States of America.
[3] While on his way there, from his own personal funds he contributed one-third of the money with which the Fathers of Mercy bought Spring Hill College (later a Jesuit institution) near Mobile, Alabama, thereby establishing their presence in the country.
His stirring eloquence brought about a religious revival which in a traditionally Catholic society which was facing apathy and competition from French-speaking Swiss Huguenots, who had begun to preach in the colony.
Some of his biographers have judged that, as an intransigent reactionary, he would prepare the way for the ultramontane clericalism that Ignace Bourget, the Bishop of Montreal would use to full advantage in both the religious and the political spheres.
[5] The high point of Forbin-Janson's apostolate in Lower Canada was the raising of an immense cross on Mont Saint-Hilaire that was a counterpart to the one on Mont-Valérien which had been destroyed.
Forbin-Janson's last visit in the United States was to Philadelphia, in November 1841, where he assisted at the consecration of Peter Richard Kenrick as coadjutor bishop of the Diocese of St.
Pope Gregory named him a Roman Count and Assistant at the Pontifical Throne "because of his wonderful zeal for the propagation and defense of the Catholic Faith in the United States of America".
In August 1842 Forbin-Janson went to London to intervene with Lord Stanley, the Secretary of State for War and the Colonies, on behalf of the Canadian rebels.