Caius (Greek Gaios, Latin Gaius) was — per the Liber Pontificalis — born at Salona in Dalmatia, the same Dalmatian city that was the birthplace of the Emperor Diocletian (the two were said to be kinsmen). He succeeded Eutychian as Bishop of Rome on December 17, 283, and governed the Roman community for nearly thirteen years — the entire critical period of the establishment of the Diocletianic tetrarchy (Diocletian was acclaimed emperor in November 284) before the outbreak of the Great Persecution.
The Liber Pontificalis attributes to him the foundational establishment of the formal cursus honorum of the Roman clerical hierarchy: that the candidate for the priesthood must first serve as ostiary (porter), then lector, then exorcist, then acolyte, then subdeacon, then deacon, then priest, then (potentially) bishop — the standard medieval Latin clerical sequence of orders that remained in use until the 1972 Vatican II reform. The historical reliability of this 6th-century attribution to a late-3rd-century pope is contested, but the persistence of the tradition through medieval canon law was substantial.
He governed during the build-up to the catastrophic Great Persecution of Diocletian (303–313) — the most systematic empire-wide Roman persecution in church history. He died peacefully on April 22, 296, before the outbreak of the great persecution. His death — the death of a kinsman of the reigning Emperor Diocletian — is one of the moments at which subsequent historical hindsight identifies the imminent reversal of the Roman attitude toward Christianity. He is buried in the Catacombs of San Callisto.