Leaving his family and possessions, Isidore retired to a mountain near the city of Pelusium, the name of which was henceforth connected with his own, and embraced the religious life in the monastery of Lychnos, where he soon became remarkable for his exactitude in the observance of the rule and for his austerities. A passage in his voluminous correspondence affords reason to believe that he held the office of abbot. He is spoken of as a priest by Facundus and Suidas, although neither of these writers informs us concerning the church to which he belonged; it may be that he had no clerical charge, but was only a priest of the monastery. His correspondence gives us an idea of his activity. It shows him fighting against unworthy clerics whose elevation to the priesthood and diaconate was a serious peril and scandal to the faithful. He complains that many laymen were ceasing to approach the sacraments so as to avoid contact with these discreditable men.
His veneration for St. John Chrysostom led him to introduce St. Cyril of Alexandria to render full justice to the memory of the great doctor. He opposed the Nestorians, and during the conflict which arose at the end of the Council of Ephesus between St. Cyril and John of Antioch, he believed there was too much obstinacy on St. Cyril's side. He therefore wrote to the latter in urgent terms imploring him, as his father and as his son, to put an end to this division and not to make a private grievance the pretext for an eternal rupture.
St. Isidore was still alive when the heresy of Eutyches began to spread in Egypt; many of his letters depict him as opposing the assertion of only one nature in Jesus Christ. It seems as though his life was scarcely prolonged beyond the year 449, because there is no mention in letters of the Robber Council of Ephesus (August, 449) nor of the Council of Chalcedon (451).
