For instance, the same approach is used to assume that Secret Mark's relationship to the canonical gospels (i.e. that it is a pastiche of canonical passages). But it could just as easily be the other way around. The key testimony is Irenaeus again:
This is the key passage again. On the one hand the passage appears in the eighth chapter of a work against the Valentinians principally. But in the parallel 'stand alone' text of Against the Valentinians by Tertullian, the material does not appear - but most everything around it does.Such, then, is their system, which neither the prophets announced, nor the Lord taught, nor the apostles delivered, but of which they boast that beyond all others they have a perfect knowledge. They gather their views from other sources than the Scriptures; and, to use a common proverb, they strive to weave ropes of sand, while they endeavour to adapt with an air of probability to their own peculiar assertions the parables of the Lord, the sayings of the prophets, and the words of the apostles, in order that their scheme may not seem altogether without support. In doing so, however, they disregard the order and the connection of the Scriptures, and so far as in them lies, dismember and destroy the truth. By transferring passages, and dressing them up anew, and making one thing out of another, they succeed in deluding many through their wicked art in adapting the oracles of the Lord to their opinions. Their manner of acting is just as if one, when a beautiful image of a king has been constructed by some skilful artist out of precious jewels, should then take this likeness of the man all to pieces, should rearrange the gems, and so fit them together as to make them into the form of a dog or of a fox, and even that but poorly executed; and should then maintain and declare that this was the beautiful image of the king which the skilful artist constructed, pointing to the jewels which had been admirably fitted together by the first artist to form the image of the king, but have been with bad effect transferred by the latter one to the shape of a dog, and by thus exhibiting the jewels, should deceive the ignorant who had no conception what a king's form was like, and persuade them that that miserable likeness of the fox was, in fact, the beautiful image of the king. In like manner do these persons patch together old wives' fables, and then endeavour, by violently drawing away from their proper connection, words, expressions, and parables whenever found, to adapt the oracles of God to their baseless fictions. We have already stated how far they proceed in this way with respect to the interior of the Pleroma.
The question is again what does 'the oracles of God' here mean? Many see it as a reference to Papias's 'oracles of the Lord' (I am one). But clearly the underlying understanding is to a pastiche or cento gospel in the hands of the heretics. The idea of changing Jesus from 'Christ' (= a king) to fox (= something sneaky and 'secret') fits the 'secret gospel' milieu. A similar reference to 'secret gospel' appears in Prescription against Heresies.