" St. Paul and St. John had begun the struggle against Gnosticism, which in their time was in its early stages. Even so, it was already showing its pernicious tendencies: promoting its seductive secret knowledge in the Christian communities, confusing simple believers, and spreading the first dangerous ‘pluralism’ within the unity of the faith. A real showdown only became possible when all the various systems of Gnosticism had been constructed. This took place towards the end of the second century. "

― Irenaeus of Lyons , The Scandal of the Incarnation: Irenaeus Against the Heresies