A Christian house-church, known as Titulus Callixti, was founded here about 220 by
Pope St. Callixtus I
(217-222) on the site of the Taberna meritoria, an asylum for retired soldiers. The area was given over to Christian use by the
when he settled a dispute between the Christians and tavern-keepers, saying, “I prefer that it should belong to those who honor God, whatever be their form of worship, was better than drunkedness and debauchery.”
Pope Julius I
(337-352) rebuilt the titulus Callixti on a larger scale, and it became the titulus Iulii commemorating his patronage, one of the original twenty-five parishes in Rome; indeed it may be the first church in which Mass was celebrated openly.
It underwent two restorations in the fifth and eighth centuries. In 1140-43 the church was re-erected on its old foundations under
Pope Innocent II
. The richly carved Ionic capitals reused along its nave were pillaged from the ruins of the Baths of Caracalla. When scholarship during the nineteenth century identified the faces in their carved decoration as Isis, Serapis and Harpocrates, a restoration under
Pope Pius IX
in 1870 hammered off the offending faces.
For reference, a plan of the church is available here