We know there was an account of baptism in their gospel because the Marcionites employed such rituals in their Church. The only point was of course that this sacrament was not derived from familiar notions of Jesus being dunked by John the Baptist as a bird came down on his head.
I am beginning to assemble the pieces for the argument that the canonical accounts of 'John the Baptist and Jesus in the Jordan' were developed ultimately from the unspoken understanding of the rich youth's initiation by Jesus in Secret Mark. My assumptions would be of course that the person being initiated in LGM 1 (= the 1st addition to the Longer Gospel of Mark) was John Mark, the patron saint of Alexandria and that the Catholic tradition developed 'John the Baptist' and his baptism from a deliberate attempt to obscure what was understood to have went on in Mark chapter 10.
The first thing that we have do is go back to what von Harnack and many others noticed about the Marcionite gospel - viz. that there was no narrative where Jesus came to a man named 'John the Baptist' in order to undergo the 'baptism of remission of sins.' John the Baptist was undoubtedly a wholly fictitious creation of the Catholic tradition to turn around the traditional Alexandrian idea of Jesus establishing John Mark as the Christ.
Let's first go beyond von Harnack suggests - the baptism of Jesus narrative was non-existent - but that 'John the Baptist' was non-existent. The first important clue is that despite 'baptizing all of Judea' the Jews themselves have never heard of such a figure outside of Christian sources. We see Tertullian also acknowledge against the Marcion that:
the heretic may blush at frustrating, to his own frustration, the mission of John [the Baptist]. For if there had been no ministry of John at all — " the voice" as Isaiah calls him, "of one crying in the wilderness," and the preparer of the ways of the Lord by denunciation and recommendation of repentance; if, too, he had not baptized (Christ) Himself along with others, nobody could have challenged the disciples of Christ, as they ate and drank, to a comparison with the disciples of John, who were constantly fasting and praying [Tertullian Against Marcion 4:11]
Moreover Celsus or his 'Jew' clearly also questioned the existence of John the Baptist for Origen writes:
I would like to say to Celsus, who represents the Jew as accepting somehow John as a Baptist, who baptized Jesus, that the existence of John the Baptist, baptizing for the remission of sins, is related by one who lived no great length of time after John and Jesus. For in the 18th book of his Antiquities of the Jews, Josephus bears witness to John as having been a Baptist, and as promising purification to those who underwent the rite. Now this writer, although not believing in Jesus as the Christ, in seeking after the cause of the fall of Jerusalem and the destruction of the temple, whereas he ought to have said that the conspiracy against Jesus was the cause of these calamities befalling the people, since they put to death Christ, who was a prophet, says nevertheless--being, although against his will, not far from the truth--that these disasters happened to the Jews as a punishment for the death of James the Just, who was a brother of Jesus (called Christ),--the Jews having put him to death, although he was a man most distinguished for his justice. [Against Celsus 1.47]
So what are we left with? The image that Celsus gives us of Jesus and John 'bathing' together in the water side by side one another. This is the start of understanding the origins of the Christian sacrament.
