Gnosticism influenced the dualistic philosophy of Docetism. See Kurt Rudolph, Gnosis: The Nature and History of an Ancient Religion, trans. Robert M. Wilson (Edinburgh: T & T Clark, 1983), 372. Countering Docetism, Irenaeus writes about Christ: "Fasting forty days, like Moses and Elias, He afterwards hungered, first, in order that we may perceive that He was a real and substantial man -- for it belongs to a man to suffer hunger when fasting; and secondly, that His opponent might have an opportunity of attacking Him" (Adv Haer 5.21.2).
Irenaeus likewise castigates the numerological tendencies of the Gnostics, observing: "Moreover, they possess no proof of their system, which has but recently been invented by them, sometimes resting upon certain numbers, sometimes on syllables, and sometimes, again, on names; and there are occasions, too, when, by means of those letters which are contained in letters, by parables not properly interpreted, or by certain [baseless] conjectures, they strive to establish that fabulous account which they have devised” (Ibid. 2.28.8).
However one defines the term "Christianity," una voce, believers of all stripes can no doubt agree with what Paul Tillich points out regarding Gnosticism: "If Christian theology had succumbed to this [Gnostic] temptation, the particular character of Christianity would have been lost. Its unique basis in the person of Jesus would have become meaningless." See Tillich, A History of Christian Thought, 36. But orthodox theologians of the church offered a successful riposte to the Gnostic challenge. These ecclesiastical polemicists "fought against gnosticism [sic] and expelled it from the church." (Ibid. 37.) In this way, the regula fidei, which the apostles supposedly transmitted to their successors, was purportedly saved.