Irenaeus formulated a four-gospel canon and an early list of authoritative New Testament books to counter Gnostic claims of secret knowledge.

To counter Gnostic teachers like Valentinus and Marcion, who claimed secret spiritual insights, Irenaeus of Lyons established a formal, public rule of faith. In his landmark work Against Heresies, he argued for the unique authority of exactly four Gospels. By anchoring Christian truth in public apostolic succession and a defined set of scriptural texts rather than private revelations, he laid the structural foundations for what would become the orthodox New Testament canon.