Since the Gnostics already knew what the text meant, they were no longer constrained by what the text "said"

This refusal to subscribe to a literal understanding of the text was a source of perennial frustration for the proto-orthodox church fathers. ... Since the Gnostics already knew what the text meant (Christ had told them!) they were no longer constrained by what the text "said" (or at least what the orthodox said it said)

B. D. Ehrman, The orthodox corruption of scripture (1993), 123

But the final phrase, "The Son of God," is lacking in several important witnesses

The vast majority of manuscripts introduce the Gospel of Mark with the words; "the beginning of the Gospel of Jesus Christ, the Son of God." But the final phrase, "The Son of God," is lacking in several important witnesses... In terms of numbers, the support for the shorter text is slight. But in terms of antiquity and character, this not a confluence to be trifled with. It frequently has been trifled with, however, and here is where one finds no little confusion in earlier discussions of the problem

B. D. Ehrman, The orthodox corruption of scripture (1993), 72

As you can probably guess, we have been reading the opening lines of Mark's gospel wrong. There's no Son of God in the original.