"Irenaeus gives an account of [the heretic] Marcus and the Marcosians in 1.13 - 21 ... Hippolytus and Epiphanius (Haer 34) copy their accounts from Irenaeus, and probably had no direct knowledge of the works of Marcus or of his sect. Clement of Alexandria, however, knew and used his writings." [Philip Schaff note on Eusebius Church History iv.11.4]
" ... for on comparison of the sections just cited from Clement and from Irenaeus [regarding the Marcosians] the coincidences are found to be such as to put it beyond doubt that Clement in his account of the number six makes an unacknowledged use of the same [Marcosian] writing as were employed by Irenaeus." [William Smith A Dictionary of Christian Biography p. 161]
"Clement of Alexandria, himself infected with Gnosticism, actually uses Marcus number system though without acknowledgement (Strom, VI, xvi)." [Arendzen JP. Marcus. The Catholic Encyclopedia, Volume IX]
The citation of the Marcosian heretical material in Stromata Book Six is often verbatim. Clement always tells us when he is citing the works of Valentinus or Basilides. Why is it that when he uses the writings associated with the heretic 'Mark' he passes them off as his own ideas or those of his Alexandrian tradition? To answer that question one needs insight, and insight comes with experience.
One can argue that Clement had a 'secret' heretical interest in the writings of a certain 'Marcus' or that his Alexandrian tradition of St. Mark was really one and the same with the figure demonized in the writings of Irenaeus. It is of course 'safe' to stick with the original proposition. No one will ridicule you. But does it really explain the evidence? This, especially when there are uncanny parallels between the description of the 'second baptism' of the followers of the heretic Mark and the rite described in the Letter to Theodore with respect to St. Mark. Both reports even connect the ritual to material associated with Mark 10:35 - 45.
But then again these people - these 'believers' - invent a crazy scenario where a reputable scholar forged a letter of Clement rather than acknowledge the existing evidence. I ask, who is the real conspiracy theorist here? The parallels are so uncanny. I don't even know why any of this is controversial. But then again, Rod is starting with faith and concluding with 'answers.' How can there be definitive answers about these things when there is so much we simply don't know? 'Believing in the Church Fathers' is a cop out. What is it that we are 'believing in' with respect to Clement? That he was orthodox? That he 'held fast' to the same beliefs that modern Christians do? Really? Clement of Alexandria?
For that you really must just close your eyes and believe ...