The Gnostic emphasis on salvation by means of a special, mystical knowledge meant that sacred Scripture would either have to be changed (as in the case of Marcion) or ignored altogether. Otherwise, it would be hard for Gnostic teachers to gain ground in the church, so different was their teaching from the truths laid forth in the Bible.
During the second century, the church was gifted with a masterful defender of biblical authority — Irenaeus, bishop of Lyons (located in present-day France). In his work Against Heresies, Irenaeus confronts the teachings of Gnosticism, appealing to apostolic tradition as the final arbiter of truth.
When we speak of apostolic tradition we are not using the term tradition to denote something that is independent of Scripture. According to biblical categories, the word tradition simply refers to something that has been handed over. For Irenaeus, tradition is simply that which has been given to the apostles and through them given to us, that is, nothing less than the Bible itself.
Gnostics appealed to hidden, unwritten teachings from the apostles for their teachings. Irenaeus countered that these do not exist because the bishops knew of no such secret sayings of Jesus or His disciples. If anyone would know of these, it would be the bishops — the successors of the apostles. Irenaeus’ argument did not rule out the possibility that there could be unwritten teachings passed on from apostles to bishops, and, unfortunately, the medieval church later developed an idea of tradition that said infallible teachings can be found also in the magisterium, the teaching office of the church. But this idea was foreign to Irenaeus, and we must keep his context in mind: he was fighting Gnostic heresy and rightly leaned on the succession of apostolic tradition.
Irenaeus is clear that this apostolic tradition is the final arbiter of truth in the church. It is impossible, he said, to appeal to some secret words of Jesus at the expense of the inscribed apostolic word, because to deny the apostles is to deny the Lord who sent them. The words of the apostles and prophets are Christ’s words and the very foundation of the church (Eph. 2:19–20).