Yes, truth is still truth. But how we construe, express, structure, interpret, explain, etc., the truth that we have grasped does change as worldviews and knowledge change. The truth of gravity is exactly what it was a thousand years ago, but how we construe, express, structure, interpret, explain that truth now is very different.
And becasue truth is truth, I think we are also bound to read the New Testament—Mark’s Gospel, for example—on its own terms. As soon as we read Mark 1 from within its historical context, on the presumption of a Jewish outlook and worldview, with the Jewish scriptures actively in mind, I think we have to recognize that the story Mark is setting out to tell is not how God became man but how Jesus became Lord.
That, to my mind, is equally a matter of truth, and I’m not prepared to sacrifice it in the interest of preserving a superficial orthodoxy. Reading later ideas into a historical text is not the right way to defend trinitarian belief. It is dishonest, but worse than that, it obscures what the story that Mark actually wanted to tell.