Pope Gregory III
San Callisto (, ) is a Roman Catholic titular church in Rome, Italy, built over the site of Pope Callixtus I's martyrdom (c. AD 222). The original building dates from the time of Pope Gregory III (r. 731–741), who ordered the building of a church on the site. The church has been rebuilt twice since, first in the twelfth century, and the current church in 1610. In 1458 Callixtus III decreed it a titular church as a seat for Cardinals.