Pope Callixtus I (Greek: Κάλλιστος), also called Callistus I, was the bishop of Rome (according to Sextus Julius Africanus) from c. 218 to his death c. 222 or 223.

[4] Emperor Julian the Apostate, writing to a pagan priest, said:[4] Christians have gained most popularity because of their charity to strangers and because of their care for the burial of their dead.In the third century, nine bishops of Rome were interred in the Catacomb of Callixtus, in the part now called the Capella dei Papi.

[citation needed] In 217, when Callixtus followed Zephyrinus as Bishop of Rome, he started to admit into the Church converts from sects or schisms who had not done penance.

[4] Hippolytus found Callixtus's policy of extending forgiveness of sins to cover sexual transgressions shockingly lax and denounced him for allowing believers to regularize liaisons with their own slaves by recognizing them as valid marriages.

The 4th-century basilica of Ss Callixti et Iuliani was rebuilt in the 12th century by Pope Innocent II and rededicated to the Blessed Virgin Mary.

The 8th-century Chiesa di San Callisto is close by, with its beginnings apparently as a shrine on the site of his martyrdom, which is attested in the 4th-century Depositio martyrum and so is likely to be historical.