Martin de Barcos (1600–1678) was a French theologian of the Jansenist School.
He was born at Bayonne, a nephew of Jean du Vergier de Hauranne, Abbot of Saint-Cyran, who sent him to Belgium to be taught by Cornelius Jansen. When he returned to France he served for a time as tutor to the son of Robert Arnauld d'Andilly and later, 1644, succeeded his uncle at the Benedictine abbey of Saint-Cyran in Berry. He did much to improve the abbey; new buildings were erected, and the library much enhanced. Unlike many commendators of his day who scarcely ever saw the abbeys over which they held authority, Barcos became an active member of Saint-Cyran, was ordained priest 1647, and gave himself up to the rigid asceticism preached by his sect.
His friendship with du Vergier and Arnauld and, through them, with Port-Royal soon brought him to the front in the debates of Jansenism. He collaborated with du Vergier in the Petrus Aurelius and with Arnauld in the book on Frequent Communion.
Of his own treatises, some bear on authority in the Church and some on the then much-mooted questions of grace and predestination. To the first class belong (1) De l'autorité de saint Pierre et de saint Paul (1645), (2) Grandeur de l'Église de Rome qui repose sur l'autorité de saint Pierre et de saint Paul (1645). (3) Éclaircissements sur quelques objections que l'on a formées contre la grandeur de l'Église de Rome (1646). These three books were written in support of an assertion contained in the book On Frequent Communion, namely: "St. Peter and St. Paul are the two heads of the Roman Church and the two are one". This theory of dual church authority, implying an equality of the two Apostles, was condemned as heretical by Pope Innocent X, in 1674 (Denzinger, Enchiridion, 965).
To the second class belong: