Of several persons of this name the most notable are St Isidore of Pelusium and St Isidore of Seville. The former, who died about the year 450, was a native of Alexandria, and spent the greater part of his life in a monastery near Pelusium, of which he is supposed to have been abbot. He was an ardent disciple and supporter of Chrysostom. The memory of his learning and piety is preserved in his Letters, of which upwards of 2000 are extant. They are chiefly expository, containing much sound instruction, and are deserving of notice for the purity of their style. They are in five books, collected into one volume fol., Gr. and Lat., Paris, 1638.

Isidore of Sevilla, surnamed Hispalensis from the country of his birth, lived between 570 and 636. About A.D. 600 he succeeded his brother Leander in the archbishopric of Sevilla, and during a long and active administration of office made his name venerable as the main pillar of the Catholic Church in Spain, the restorer and maintainer of clerical discipline, the most learned and the most eloquent doctor of his age. Of his voluminous works the most remarkable is his Origines seu Etymologiorum libri xxc., an encyclopaedic collection of treatises on all the arts and sciences at that time embraced within the range of human study. The best edition of his collected works is that of Arevali, 7 vols. 4to, Rome, 1797-1803.

A celebrated collection of forged canons or decretals, intended to prove the paramount authority of the Roman See over all other tribunals, was the work of Isidore Mercator or Peccator, a monk of the eighth century.