Idelphonsus of Toledo was of noble lineage and most likely of pupil of the famous St. Isidore of Seville. He made rapid progress in holiness and became abbot of the Benedictine monastery of Toledo at a young age, in which capacity he attended the famous Councils of Toledo in 653 and 655 that are remembered for their Trinitarian definitions against the Visigothic Arians, definitions which are quoted today in the Catechism of the Catholic Church.
Idelphonsus was elevated to the episcopacy of Toledo in 657; according to legend, he had a vision of the Virgin Mary presenting him with a chalice, which became a popular subject in medieval Spanish art. He became embroiled in a local controversy over whether the Catholic dead who had not had the benefit of Last Rites could have Masses said on their behalf; St. Isidore had argued in the affirmative, but St. Idelphonsus and some of the other ecclesiastics of Toledo had objected. The Church has subsequently adopted the more moderate position of Isidore.
Idelphonsus is remembered mostly for two works: one in defense of our Lady's perpetual virginity, which is based on St. Jerome's famous treatise; another on the history of the Spanish Church. He may also be the author of the text of the Visigothic Mass of the Ascension. Idelphonsus died in 667. His Feast Day is celebrated on January 23rd.