Saint Ildelfonso lived at a time when Islamic expansion into the Iberian Peninsula was underway. The Muslims conquered Hispania in 711, just a few decades after his death, leading to significant changes in the region’s cultural and religious landscape. So he gives us a glimpse into the life of the church in Spain before the conquest.

He was born to a prominent Visigothic family in Toledo in 607AD and was educated by his uncle Eugenius, who later became Toledo's bishop, and in 632AD ordained him deacon. However, Ildefonso defied his family's plans for his clerical career and instead became a monk at the Agali monastery outside of the city. In 650 he was elected abbot of Agali and attended two synods of the Iberian church, the eighth and ninth Councils of Toledo. When his uncle Bishop Eugenius died in 657, Ildefonso was elected his successor as bishop of Toledo.

It was reported that on 18 December 665, Ildelfonso experienced a vision of the Blessed Virgin who appeared to him in person to reward him for his zeal in honouring her. Light engulfed the church, and Mary, sitting on the episcopal throne, praised him for his devotion and presented him with a special chasuble which she instructed him to wear only during Marian festivals. In part because of the vision, Rome later elevated Toledo's status to the metropolitan see of Carthaginiensis, with the most authority in the Iberian Peninsula. To this day, the Archbishop of Toledo is the Primate of Spain.

Ildefonso died on 23 January 667AD (now his feast day) and was buried at the city's basilica, Toledo's Church of Santa Leocadia. He is best known a scholar and theologian, and, although his writings were less influential outside of Iberia, he was canonised and remained a potent force in the Iberian Peninsula for centuries.

He is patron saint of Toledo and his intercession is often sought for protection against floods and other water-related disasters, which is attributed to a legendary story where the Virgin Mary saved him from drowning. Additionally, he is invoked as a patron of theologians and scholars due to his contributions to theological writings.