St. Irenaeus of Lyons, author of Against Heresies (c. 130 - c. 200 a.d.)
In the course of the discussion about the Gospel of Judas, repeated mention has been made of the fact that St. Irenaeus mentions this Gnostic Gospel in his treatise "Against Heresies" [1:31:1]:
"Others again declare that Cain derived his being from the Power above, and acknowledge that Esau, Korah, the Sodomites, and all such persons, are related to themselves. On this account, they add, they have been assailed by the Creator, yet no one of them has suffered injury. For Sophia was in the habit of carrying off that which belonged to her from them to herself. They declare that Judas the traitor was thoroughly acquainted with these things, and that he alone, knowing the truth as no others did, accomplished the mystery of the betrayal; by him all things, both earthly and heavenly, were thus thrown into confusion. They produce a fictitious history of this kind, which they style the Gospel of Judas."
Little mention has been made however of St. Irenaeus primary arguments against the Gnostics. The Gnostics were a religious movement with some similarities to the New Age movement of today, and this movement predated the time of Christ, and has no origins whatsoever in the Hebraic tradition. Gnostic, like New Agers, like to use elements of other religions as window dressing, for their non-Christian beliefs, and when they encountered Christianty, some of them began to use Christian window dressing. They claimed to have a secret tradition from Christ, but different Gnostic gospels claimed that this source came via different routes. However, what they all had in common was that there was no historical evidence that they could produce to substantiate their claims to having any connection with Christ. Here is the gist of St. Irenaeus' refutation of these claims:
"As I said before, the Church, having received this preaching and this Faith, although she is disseminated throughout the whole world, yet guarded it, as if she occupied but one house. She likewise believed these things just as if she had but one soul and one and the same heart; and harmoniously she proclaims them and teaches them and hands them down, as if she possessed one mouth. For, while the languages of the world are diverse, nevertheless, the authority of the Tradition is one and the same. Neither do the Churches among the Germans believe otherwise or have another Tradition, nor do those among the Iberians, nor among the Celts, nor away in the East, or in Egypt, nor in Libya, nor those which have been established in the central regions of the world. But just as the sun, that creature of God, is one and the same throughout the whole world, so also the preaching of the Truth shines everywhere and enlightens all men who desire to come to a knowledge of the Truth. Nor will any of the rulers in the Churches, whatever his power of eloquence, teach otherwise, for no on is above the Teacher; nor will he who is weak in speaking subtract from the Tradition. For the Faith is one and the same, and cannot be amplified by one who is able to say much about it, nor can it be diminished by one who can say but little" [Against Heresies 1:10:2]."
"It is within the power of all, therefore, in every Church, who may wish to see the truth, to contemplate clearly the tradition of the apostles manifested throughout the whole world; and we are in a position to reckon up those who were by the apostles instituted bishops in the Churches, and [to demonstrate] the succession of these men to our own times; those who neither taught nor knew of anything like what these [heretics] rave about. For if the apostles had known hidden mysteries, which they were in the habit of imparting to "the perfect" apart and privily from the rest, they would have delivered them especially to those to whom they were also committing the Churches themselves. For they were desirous that these men should be very perfect and blameless in all things, whom also they were leaving behind as their successors, delivering up their own place of government to these men; which men, if they discharged their functions honestly, would be a great boon [to the Church], but if they should fall away, the direst calamity.
Since, however, it would be very tedious, in such a volume as this, to reckon up the successions of all the Churches, we do put to confusion all those who, in whatever manner, whether by an evil self-pleasing, by vainglory, or by blindness and perverse opinion, assemble in unauthorized meetings; [we do this, I say, ] by indicating that tradition derived from the apostles, of the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church, on account of its pre- eminent authority, that is, the faithful everywhere, inasmuch as the apostolical tradition has been preserved continuously by those [faithful men] who exist everywhere.
The blessed apostles, then, having founded and built up the Church, committed into the hands of Linus the office of the episcopate. Of this Linus, Paul makes mention in the Epistles to Timothy. To him succeeded Anacletus; and after him, in the third place from the apostles, Clement was allotted the bishopric. This man, as he had seen the blessed apostles, and had been conversant with them, might be said to have the preaching of the apostles still echoing [in his ears], and their traditions before his eyes. Nor was he alone [in this], for there were many still remaining who had received instructions from the apostles. In the time of this Clement, no small dissension having occurred among the brethren at Corinth, the Church in Rome despatched a most powerful letter to the Corinthians, exhorting them to peace, renewing their faith, and declaring the tradition which it had lately received from the apostles, proclaiming the one God, omnipotent, the Maker of heaven and earth, the Creator of man, who brought on the deluge, and called Abraham, who led the people from the land of Egypt, spake with Moses, set forth the law, sent the prophets, and who has prepared fire for the devil and his angels. From this document, whosoever chooses to do so, may learn that He, the Father of our Lord Jesus Christ, was preached by the Churches, and may also understand the apostolical tradition of the Church, since this Epistle is of older date than these men who are now propagating falsehood, and who conjure into existence another god beyond the Creator and the Maker of all existing things….
But Polycarp also was not only instructed by apostles, and conversed with many who had seen Christ, but was also, by apostles in Asia, appointed bishop of the Church in Smyrna, whom I also saw in my early youth, for he tarried [on earth] a very long time, and, when a very old man, gloriously and most nobly suffering martyrdom, departed this life, having always taught the things which he had learned from the apostles, and which the Church has handed down, and which alone are true. To these things all the Asiatic Churches testify, as do also those men who have succeeded Polycarp down to the present time,-a man who was of much greater weight, and a more stedfast witness of truth, than Valentinus, and Marcion, and the rest of the heretics. He it was who, coming to Rome in the time of Anicetus caused many to turn away from the aforesaid heretics to the Church of God, proclaiming that he had received this one and sole truth from the apostles,-that, namely, which is handed down by the Church. There are also those who heard from him that John, the disciple of the Lord, going to bathe at Ephesus, and perceiving Cerinthus within, rushed out of the bath-house without bathing, exclaiming, "Let us fly, lest even the bath-house fall down, because Cerinthus, the enemy of the truth, is within." And Polycarp himself replied to Marcion, who met him on one occasion, and said, "Dost thou know me? ""I do know thee, the first-born of Satan." Such was the horror which the apostles and their disciples had against holding even verbal communication with any corrupters of the truth; as Paul also says, "A man that is an heretic, after the first and second admonition, reject; knowing that he that is such is subverted, and sinneth, being condemned of himself." There is also a very powerful Epistle of Polycarp written to the Philippians, from which those who choose to do so, and are anxious about their salvation, can learn the character of his faith, and the preaching of the truth. Then, again, the Church in Ephesus, founded by Paul, and having John remaining among them permanently until the times of Trajan, is a true witness of the tradition of the apostles" [3:3:1-4].
"When, therefore, we have such proofs, it is not necessary to seek among others the Truth which is easily obtained from the Church. For the Apostles, like a rich man in a bank, deposited with her most copiously everything which pertains to the Truth, and everyone whosoever wishes draws from her the drink of life. For she is the entrance to life, while all the rest are thieves and robbers. That is why it is surely necessary to avoid them, while cherishing with the utmost diligence the things pertaining to the Church, and to lay hold of the Traditions of Truth. What then? If there should be a dispute over some kind of question, ought we not have recourse to the most ancient Churches in which the Apostles were familiar, and draw from them what is clear and certain in regard to that question? What if the Apostles had not in fact left writings to us? Would it not be necessary to follow the order of Tradition, which was handed down to those whom they entrusted the Churches?" [3:4:1].
In short, the teachings of the Orthodox Church have a historical record that can be examined, and St. Irenaeus could say "I was taught the faith by St. Polycarp, and he was taught it by the Apostle John himself". The Gnostics had nothing but claims and assertions without any evidence to support their claims -- and this in the face of overwhelming evidence that their teachings had no roots at all in the Judeo-Christian Tradition.