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.
- Matthew Bunson, OSV’s Encyclopedia of Catholic History, (Huntington, IN:Our Sunday visitor, 2004), p. 451
- Charles A. Coulombe, Vicars of Christ: A History of the Popes (NY: Citadel Press, 2003), p. 27, 28
- Richard P. McBrien, Lives of the Popes: The Pontiffs from St. Peter to John Paul II, (San Francisco:Harper San Francisco, 1997), p. 38