After considering the evidence, the main objection we might raise is to ask if we are asking too much of the author of Mark, when we suppose he is giving us a consistent portrait of Jesus as divine? Are we not supposing Mark to believe something too hard to believe at that time; too much a product of later theological development—the divinity of Jesus as well as some notion of a Trinity?
Do these conclusions require us to suppose Mark thought in Trinitarian terms? No.
The New Jerome Biblical Commentary points out that the New Testament show us the early Christians’ terminology had to catch up with the identity they grasped.
In any case, we can collect all the "raw material" for the Trinitarian faith in Mark—we could, if we liked, proceed next to cite references to the Spirit to add to our results here—regardless of what we conclude about Mark’s own thinking.
So we need not presuppose Mark had a Nicean concept of Christ's divinity, or of the trinitarian nature of God; nor need we exclude it. What we have is his work, and the abundance of textual evidence portraying Christ as divine. To what extent this arose out of some sort of systematic thinking, or out of intuition--or dare we say, inspiration?--I think we can only speculate. But here's the thing: the more we lean away from Mark being systematic, and "ahead of his times," the more we must credit his witness. To use a stereotypical contrast: did he paint a portrait interpreting his subject; or, if he was unequipped to do that, then he photographed him, as it were, and the astonishing portrait comes "through" the camera, from the subject Himself.
With that said, we might as well point out the obvious here—that, other than the text of Mark’s Gospel, we have no idea what Mark himself believed about Jesus and his relations to the Father and the Spirit.
For that very reason, I began with the text itself—the more consistent and more pronounced is the depiction of Jesus’ divinity, then the stronger is the case that Mark himself held such a view.
Of course, we might avoid that conclusion a couple of ways. We might suppose that our human author produced a text without understanding what he wrote; or we might suppose that many sets of hands produced this text, with as many distinctive theologies and purposes, and thus the only unifying theology and purpose behind the text lies solely with the divine author. But these alternatives require more suppositions than what I propose—an author (or redactor of multiple sources) with a singular purpose, which the text he produced reveals.
But we can approach the question another way.
The human author of the Gospel of Mark had some Christology—and whatever Christology it was, it has to be fitted somehow with the monotheism of Judaism.
As the text of Mark itself shows, any Christology that has Jesus being anything more than merely human is going to be a rough fit: the scribes in Chapter 2, and the chief priest in Chapter 14, as well as lots of people in between, ask essentially the same question: "Who does this man think he is?"
Anything more than mere humanity is offensive, and thus any Christology that situates Jesus somewhere short of full divinity is no less a leap than the highest possible Christology.
This is the insight of Richard Bauckham, who argues that the "highest possible Christology" undergirds every New Testament document. Indeed, Bauckham argues that only the highest possible Christology can be reconciled with Jewish monotheism—alternative christologies in which Jesus occupies "ambiguous or semi-divine status, participating in divinity in some subordinate way," are the least reconcilable with Jewish monotheism.
"What Jewish monotheism could not accommodate were precisely semi-divine figures, subordinate deities, divinity by delegation or participation," Bauckham argues. Including Jesus in the divine identity was unprecedented —and the Gospel texts themselves bear witness to the scandal of doing so—but "it was not a step which could be, as it were, approached gradually by means of ascending christological beliefs"—because the gulf between the Creator and creatures was absolute and yawning in Jewish monotheism—either Jesus is on our side of it, or God’s side.
The image of a canyon is apt. Pointing out how wide the chasm is, and hence how improbable a leap is required to get to the other side, only demonstrates all the more that anyone who gets to the other side manifestly did not do so by steps, but in one, fantastic, leap.