Barcos was born at Bayonne, a nephew of Jean du Vergier de Hauranne, the commendatory abbot of the Abbey of Saint-Cyran in the Duchy of Berry, who sent him to Belgium to be taught by Cornelius Jansen.
Unlike many commendatory abbots of his day, however, who scarcely ever saw the monasteries over which they held authority, Barcos became an active member of the abbey, became a priest in 1647, and gave himself up to the rigid asceticism preached by his sect.