Pope St. Callistus succeeded St. Zephyrinus. The institution of the ember-day fasts is ascribed to him. He suffered martyrdom under Emperor Alexander Severus, October 14, 233. He was a staunch defender of the unity of the Divine Essence. He offered absolution to even the greatest sinner who should have preformed the canonical penances.
(Source: The New Roman Missal, Rev. F. X. Lasance)