The Gospel of Peter also goes by the name the Recollections of Peter, and Justin Martyr often resorts to the word ‘memoir’ in place of ‘gospel’; whereas many of his quotations are in fact said to have originated in the Gospel of Peter. In the Dialogue (around 160 AD), Justin furthermore states: “The mention of the fact, that Christ changed the name of Peter, one of the apostles, and that the event has been recorded in his (Peter’s) Memoirs…”
Origen (230 AD), in his commentary on Matthew, also mentions the Gospel of Peter: “There are some”, says he, “who say the brethren of Christ were the children of Joseph, by a former wife, who lived with him before Mary; and they are adduced to this opinion by some passages in that which is entitled the The Gospel of Peter…”
You seem to think that the notions to do with Jesus sprang up out of nowhere some time in second century, whereas they evolved over some period of time prior thereto. As mentioned previously, even the Gospel of the Hebrews (possibly originating a few year prior to 125 AD), comprised a compilation of manuscripts already in existence.
You speak of a ‘Jesus cult’, but which Jesus and what cult?
I already mentioned the Gnostic-styled spiritualism of Paul and Clement, whereas there’s no evidence that either of the two gospels in question taught the immaculate conception, or the material resurrection of Christ, or contained any account of his miracles, or made any reference to any other writing containing such elements. In fact, the event that Christ was begotten of a virgin through the agency of the Holy Ghost doesn’t appear to receive a mention in heathen, Christian, or Jewish history until more than a century after it is alleged to have taken place.
Many sources no doubt refute Tertullian’s ‘Against Marcion’, yet I fail to see how this dispels the fact that he did mention or authenticate the existence of a gospel by Peter.
Prior to Marcion’s compilation (embracing ten of Paul’s Epistles), it’s doubtful whether Paul’s Epistles were generally well known or accepted. As mentioned, Paul was rejected by most of the Jewish Christians. I believe that the early German scholars whittled the number of genuine ones down to four, whereas the Dutch Radicals cast doubt on Galatians and Romans.
Justin Martyr doubtlessly had his own reasons for not mentioning Paul or of making little of Marcion’s revealed Paul’s Epistles, even though Wikipedia does state: “Reflecting his opposition to Marcion, Justin's attitude toward the Pauline epistles generally corresponds to that of the later Church. In Justin's works, distinct references are found to Romans, Corinthians, Ephesians, Colossians, and 2 Thessalonians…”
http://en.wikipedia.org/wiki/Justin_Martyr As for Justin Marty, many others no doubt also purposely or otherwise failed to mention Paul, or his works, but this hardly does away with Marcion’s publication, or the part played in the formation of the Christian religion.
Wikipedia on the Muratorian Canon: “The unidentified author accepts four Gospels, the last two of which are Luke and John, but the names of the first two at the beginning of the list are missing. Also accepted by the author are the Acts of the Apostles and 13 of the Pauline Epistles (the Epistle to the Hebrews is not mentioned in the fragment). The author considers spurious the letters claiming to have Paul as author that are ostensibly addressed to the Laodiceans and to the Alexandrians. Of these he says they are "forged in Paul's name to [further] the heresy of Marcion."
http://en.wikipedia.org/wiki/Muratorian_fragment