Of course when we look carefully Clement does put forward the Marcionite concept of the 'baptism on behalf of the dead' but notice HOW HE DOES IT. He cites that most Roman text of the New Prophesy movement, the Shepherd of Hermas (before the whole tradition fell out of fashion).
In other words, Clement has found a way to 'cover his ass' when he confesses the secret Marcionite beliefs of the Alexandrian community - make it seem as if he is citing 'authorized Scripture.' So we read:
And the Shepherd, speaking plainly of those who had fallen asleep, recognises certain righteous among Gentiles and Jews, not only before the appearance of Christ, but before the law, in virtue of acceptance before God, -- as Abel, as Noah, as any other righteous man. He says accordingly, "that the apostles and teachers, who had preached the name of the Son of God, and had fallen asleep, in power and by faith, preached to those that had fallen asleep before." Then he subjoins: "And they gave them the seal of preaching."
They descended, therefore, with them into the water, and again ascended. But these descended alive, and again ascended alive. But those, who had fallen asleep before, descended dead, but ascended alive. By these, therefore, they were made alive, and knew the name of the Son of God. Wherefore also they ascended with them, and fitted into the structure of the tower, and unhewn were built up together; they fell asleep in righteousness and in great purity, but wanted only this seal." [Stromata Book II.9]
And again:
And it is well said by the Shepherd, "They went down with them therefore into the water, and again ascended. But these descended alive, and again ascended alive. But those who had fallen asleep, descended dead, but ascended alive." Further the Gospel says, "that many bodies of those that slept arose," -- plainly as having been translated to a better state. There took place, then, a universal movement and translation through the economy of the Saviour. [ibid VI.6]
In other words, no one should be surprised that Clement doesn't make explicit confession of beliefs associated with the heretical Markan tradition. He's still working on it, or was until the day he dies, trying to find some clever way of hiding it inside some confession of Orthodoxy.
In this Origen certainly did improve upon his master. He took this methodology to a whole new level, and the entire Alexandrian community was grateful for his efforts ...
If you are interested in reading how this observation fits within my greater understanding of the workings of Secret Mark WITHIN the contemporary Alexandrian Church please go here
If you want to read more about how Alexandrian Christianity was rooted in the Jewish traditions of Alexandria, Philo of Alexandria and more feel free to purchase my new book here
