Cullen starts off by describing God in conventional terms as "good, well-meaning, kind," but then progresses to question God. He states that he realizes that, as a human, he is like a blind mole who simply doesn't have the capacity to understand God's "inscrutable" ways. Nevertheless, this doesn't stop him wondering about God's behavior.
He asks, for example, why a good God would punish Tantalus, who, in Greek mythology, tried to steal the nectar and ambrosia of the gods so that he could give it to his people. For this, he was forced to stand in water under a fruit tree. Every time he tried to drink the water it would recede, and every time he reached for fruit it would raise itself out of his reach. The poet also wonders why God punished Sisyphus, the character from Greek mythology who was forced to push a large rock up a hill only to have it roll down again near the top so that he had to start over.
As we read the poem, we might being to question the opening assertion that God is "good, well-meaning, kind." The images of seemingly endless and pointless punishment suggest that God is cruel, not kind.
As the speaker thinks about God, he comes to describe him as "awful," not kind, and repeats the word "awful" twice for emphasis. He writes:
What awful brain compels His awful hand.
We should keep in mind that Cullen is using an older meaning of awful, which doesn't simply mean terrible but also encompasses the idea of awe-inspiring. Nevertheless, "awful" shows a shift in attitude toward God from the gentle adjectives "kind" and "good."
Finally, Cullen ends with the question of why God would
make a poet black, and bid him sing!
Given the allusions to Sisyphus and Tantalus, we are led to question whether God is being kind or cruel in giving a poet black skin in a racist society. Is the poet being punished like the two characters in Greek mythology by being given a hopeless task?
So yes, the speaker's attitude does shift to become more questioning and critical toward God over the course of the poem.