So, what's all this marveling about? The speaker spends the first 12 lines throwing his hands up in the air and saying, "I don't get it!" Marvel, my foot. Sure, he says, God is good and could probably explain all of the things that make me go hmmm, but still, there are things in the world that he'll just never get.
Although the first 12 lines of the poem address the universal theme of a good God that allows suffering in the world, the last two lines spin the poem's focus into marveling at how God could make "a poet black." Although stereotypes during the time minimized African Americans' ability to make worthwhile contributions to something as highbrow as art in America, this speaker finds a reason to marvel at his identity as both a poet and an African American.