Denault's death in 1806 gave Lartigue the freedom to become a member of the Society of Saint-Sulpice, into which he was received in February of that year, seeking a more contemplative and intellectual life. He joined the Sulpician community of the Seminary, the first native Canadian to enter the Society. He was assigned to help in the Parish of Notre-Dame which was attached to the Seminary. That same year, Denault's successor, Joseph-Octave Plessis, called upon his legal knowledge to challenge an effort by Attorney General Jonathan Sewell to challenge the legal standing of Catholic parishes created since the British conquest of New France.