Callinicus was a 3rd-century martyr from Bithynia who underwent a significant religious transformation. Originally serving as a pagan priest, he was moved to embrace Christianity after witnessing the bravery shown by the martyr Phyrrhus. His conversion eventually led to his death during the persecutions led by Decius, which occurred between 249 and 251. He is recognized as a saint within both the Catholic and Orthodox traditions.