"The first execution of a heretic by Christians occurred in Spain, when Bishop Priscillian of Avila was executed. The trial of Priscillian and other heretics set a new precedent. It was the first time a secular court had convicted Roman citizens on religious grounds. Priscillian was then denied a hearing by Bishop [St.] Ambrose at Milan and by Pope [St.] Damasus I at Rome. He appealed to Emperor Maximus at Trier and was sentenced to death. [McCabe, 1939, 86; Catholic Encyclopedia, "St. Ambrose" and "Inquisition"; Engh, 98]"
Catholic Encyclopedia, "St. Ambrose" and "Inquisition"
McCabe, 1939, 86