Isidore of Pelusium, Saint, born at Alexandria in the latter half of the fourth century; d. not later than 449-50. He is occasionally designated through mistake as Isidore of Damietta. 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 induce 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 his letters of the Robber Council of Ephesus (August, 449) nor of the Council of Chalcedon (451).
According to Evagrius, St. Isidore was the author of a great number of writings, but this historian tells us nothing further, save that one of these was addressed to Cyril, even leaving us ignorant whether this person was the celebrated Bishop of Alexandria or a namesake. Isidore himself tells incidentally that he composed a treatise "Adversus Gentiles", but it has been lost. Another work "De Fato", which, the author tells us, met with a certain degree of success, has also been lost. The only extant works of St. Isidore are a considerable correspondence, comprising more than 2000 letters. Even this number appears to fall far short of the amount actually written, since Nicephorus speaks of 10,000. Of these we possess 2182, divided into five books which contain respectively 590, 380, 413, 230, and 569 letters. These letters of St. Isidore may be divided into three classes according to the subjects treated: those dealing with dogma and Scripture, with ecclesiastical and monastic discipline, and with practical morality for the guidance of laymen of all classes and conditions. Many of these letters, as is natural, have but a secondary importance, many are mere notes. In this article attention can be drawn only to the principal ones. Among these is the letter to Theologius against the Nestorians, in which Isidore points out that there is this difference between the mother of the gods in fable and the Mother of Jesus Christ, the Son of God, that the former, as acknowledged by the pagans themselves, conceived and brought forth the fruits of debauchery, whereas the latter conceived without having had intercourse with any man, as is acknowledged, says he, by all the nations of the world. His letter to Hierax defends the legitimacy of the veneration of relics; that to Tuba shows that it was considered unbecoming for a soldier to carry a sword in the city in time of peace and to appear in public with arms and military uniform.
His letters addressed to persons following the religious life afford many important clues which enable us to form a fairly exact idea of the intellectual standard then existing in Egyptian monastic centers. Isidore reproaches the monk Thalebeus with being interested in reading pagan historians and pagan poets which were full of fables, lies, and obscenities capable of opening wounds that had healed and of recalling the spirit of uncleanness to the house from which it had been ejected. His advice with regard to those who were embracing the monastic state was that they should not at first be made to feel all the austerities of the rule lest they should be repelled, nor should they be left idle and exempt from ordinary tasks lest they should acquire habits of laziness, but they should be led step by step to what is most perfect. Great abstinences serve no purpose unless they are accompanied by the mortification of the senses. In a great number of St. Isidore's letters concerning the monastic state it may be remarked that he holds it to consist mainly in retirement and obedience; that retirement includes forgetfulness of the things one has abandoned and the renunciation of old habits, while obedience is attended with mortification of the flesh. A monk's habit should if possible be of skins, and his food consist of herbs, unless bodily weakness require something more, in which case he should be guided by the judgment of his superior, for he must not be governed by his own will, but according to the will of those who have grown old in the practice of the religious life.
Although for the most part very brief, the major ity of St. Isidore's letters contain much instruction, which is often set forth with elegance, occasionally with a certain literary art. The style is natural, unaffected, and yet not without refinement. The correspondence is characterized by an imperturbable equability of temperament; whether he is engaged at explaining or reprimanding, at disputing or praising, there is always the same moderation, the same sentiments of sincerity, the same sober taste. In the explanation of the Scripture the saint does not conceal his preference for the moral and spiritual sense which he judges most useful for those who consult him. Everywhere he is seen to put in practice the maxims he teaches to others, namely that the life should correspond with the words, that one should practice what one teaches, and that it is not sufficient to indicate what should be done, if one does not translate one's maxims into action.