Isidore was the youngest of four siblings who all became saints — with his elder brother Leander, his brother Fulgentius, and his sister Florentina. Leander, Bishop of Seville before him, taught him; Isidore succeeded Leander as bishop around 600 and ruled the see for thirty-six years.
He completed the conversion of the Visigothic kingdom of Spain from Arianism to Catholicism, reformed the liturgy and clerical education, and presided over the Fourth Council of Toledo (633), which established the discipline of seminaries on principles still followed today — universal Bible study, uniform liturgy, regular synods.
His Etymologiae — a vast twenty-book encyclopedia gathering the surviving knowledge of the classical world under the heading of the etymology of each word — was the standard reference of the European Middle Ages for nine hundred years. Over a thousand medieval manuscript copies survive. He is the last writer of antiquity. Pope John Paul II proposed him as patron of the Internet in 1999, recognising the encyclopedic ambition of his work.