Adoptionism, Heresy and Secret Mark

Adoptionism - the belief that Christ was adopted as God's son at the time of his baptism - is supposed to be a heresy. It was the reason Paul of Samosata was condemned. Yet there is something to the logic that if Christians are baptized 'into Christ' that, on some level, the guy who is now called 'Christ' also must have undergone this same process. Why else reference a baptism narrative in the gospel? The liturgy must be grounded in the 'acts of Christ.' What else is the function of the gospel?

Indeed a number of passages in the Apostolikon sound distinctly adoptionist like the superscription in Romans, where Paul is said to have been:

called to be an apostle and set apart for the gospel of God — the gospel he [God] promised beforehand through his prophets in the Holy Scriptures regarding his Son, who as to his earthly life was a descendant of David, and who through the Spirit of holiness was appointed the Son of God in power by his resurrection from the dead [Romans 1:2 - 4]

I have previously noted that the superscription at the beginning of Galatians in its original Marcionite form makes it sound as Paul was the Christ resurrected from the dead. I wonder again if the Marcionite apostle was the one who was adopted in the narrative from Secret Mark mentioned in the Letter to Thedoore. We must always remember that the Marcionite gospel did not have the baptism of Jesus by John the Baptist (or anyone else for that matter).