That office, the second-highest in the Order, required that he reside in Rome, where, according to the Code of Canon Law then in effect and the then-current General Constitutions of the Order, his special duties were to conduct all dealings of the Order with the various dicasteries of the Holy See.

A friar of exceptionally varied administrative experience, Father Mathias had been Minister Provincial of Holy Name Province, with headquarters in New York, for four terms (1919-22, 1922-25, 1931-34, 1934-37). During the years ofWorld War II (1942-46) he had held the office of Delegate General for North America. At that time he had fostered the search for a selected friar or group of friars from among all the martyrs to be proposed for the honors of the altars. Now in his Roman post of greater authority and prestige, he was in an advantageous position to perceive the spirit and the practice of the Holy See in handling Causes looking to the canonization of Saints. He was thus in a favorable spot to orient the choice of which, from the many possible Servants of God, should be selected and proposed first.

With the consent of the General Definitorium of the Order of Friars Minor, acting in the name of Father Pacifico Perantoni, Minister General, and in his own capacity as Procurator General of the whole Order, on July 16, 1950, Father Mathias appointed Father Marion Habig to the position of Pro-Postulator for the Cause of the Five Franciscan Martyrs of Georgia. In pursuit of that objective, in September of that same year the Postulator General of the Order, Father Fortunato Scipioni, set up a Commission to collect all available documents which would aid the Holy See in the eventual judgment of the Cause. The three members of the Historical Commission were (in Spain) Father Ignacio Omaechevarria, and (in the United States) Father Marion Habig, resident in St. Louis, and Father Alexander Wyse, Director of the Academy of American Franciscan History in Washington, D.C.

The search for and the investigation of the Spanish sources was committed to Father Omaechevarria, a researcher of renown, whose popular Sangre Viscaína en los Pantanos de La Florida (Victoria, 1947), while focusing specifically on the Basque friar who was one of the Five, had already publicized the story of these Spanish missionaries of the sixteenth century. In the Archives of the Indies, Seville, he examined the original documents dating from the era of the actual slaughter; most of these had already been transcribed by American researchers and were available to students in the form of photostats or typescripts in the Manuscript Division of the Library of Congress (Stetson Collection) in Washington, D.C. Father Omaechevarria's patient study of the pertinent sixteenth and seventeenth century sources was published in a critical edition under the title Mártires Franciscanos de Georgia. Informes Relaciones sobre su Muerte (Madrid, 1955). In addition, these documents with Omaechevarria's commentary were issued in the scholarly journal Missionalia Hispanica Vol. XII, nos. 34 and 35 (1955) of the Consejo Superior de Investigaciones Cientificas, Madrid.

In November of 1957, at the annual meeting of the American Hierarchy, Archbishop John M. Gannon, of Erie, submitted a proposal that the bishops petition the Holy See for a favorable acceptance of the Cause of the Five Martyrs of Georgia, as in 1884 the Fathers of the III Plenary Council of Baltimore had done in the Cause of the Eight Jesuit Martyrs of North America (eventually beatified in 1925 and canonized in 1930).

With the zeal and capacity which had made him an outstanding promoter of things Franciscan, during the next twenty to twenty-five years, Father Marion labored indefatigably to make the Cause known. He issued an historically accurate and attractive brochure, containing a prayer authorized by Most Rev. Raymond W. Lessard, Bishop of Savannah, for their Beatification. Similarly he published many articles on the topic of martyrdom in different magazines. He listed these Servants of God in the appendix of his book, Saints of the Americas (Our Sunday Visitor, 1974). With the exception of a brief period in 1976-77, when a priest of the Atlanta Archdiocese, Rev. Richard J. Lopez, was given the office, Father Marion continued to hold the title and discharge the duties of Vice Postulator. Father Habig, to whom the Cause owes a debt of gratitude, passed to his eternal reward on November 22, 1984.