Pope Callixtus I, also called Callistus I, was the bishop of Rome from c. 218 to his death c. 222 or 223. He lived during the reigns of the Roman emperors Elagabalus and Alexander Severus. Eusebius and the Liberian catalogue list his episcopate as having lasted five years (217–222). In 217, when Callixtus followed Zephyrinus as Bishop of Rome, he started to admit into the Church converts from sects or schisms. He was killed for being Christian and is venerated as a saint by the Catholic Church.

Pope Zephyrinus was the bishop of Rome from 199 to his death on 20 December 217. He was born in Rome, and succeeded Victor I. Upon his death on 20 December 217, he was succeeded by his principal advisor, Callixtus I. He is known for combating heresies and defending the divinity of Christ.