Pope Hyginus
He served as Pope during the heretical teachings of the Gnostics. Heresy is a belief or opinion that does not agree with the official belief or opinion of a particular religion.  The Gnostics taught that Yahweh, (the Old Testament name for God) and Jesus Christ were two different Gods. This was against the Catholic Church teachings. Hyginus declared these men, Valentinus and Cerdo, to be heretics.