Uncertainty is at the heart of "Yet Do I Marvel." In fact, the second word of the poem is "doubt." Even though he says, "I doubt not," the word (it falls on the stressed syllable of the first metrical foot, so it must really matter) rings in the first line. He continues this clever wordplay throughout, always choosing words that will reinforce the uncertainty of the speaker.
Line 1: Like we said, we start off with a stress on "doubt." So although the speaker is saying he believes God is good, the presence of the word doubt gets us thinking the poem might involve a little uncertainty. He could have just said, "I know God is good…" right? Hmmm, maybe he wants the language to reinforce the mental state of being unsure which exists in the speaker.
Line 2: Here, God could "quibble." Again, we're talking about being misled and uncertain. Although the line is about God explaining uncertainty away (if He wanted to, which He doesn't) the word choice reinforces that idea of trying to figure something out and not knowing the real answer. It's subtle, but it's there, and that's why Cullen is considered such a sonnet superstar.
Line 4: This line opens with "Why." Just like the speaker is trying to figure out the reason bad things exist in the world, starting off a line with "why" is like an echo of the speaker's uncertainty. Not to mention it rhymes with the last word of the line, "die." Check out "Form and Meter" for a review of how rhyme works in this poem, but we just thought we'd chuck in a quick reminder that Cullen is using language that sonically evokes the speaker's uncertainty.
Line 6: Tantalus is "baited." Not like a hook, but this word hangs in the poem as another implication of uncertainty. The idea of being baited by life or God or suffering, or anything at all, implies deception and uncertainty, which is beginning to seem like a central idea of the speaker's message.
Line 6-7: "Fickle" and "Caprice" are all about changing your mind. If that's not uncertainty, show us what is. Or don't show us. Okay, show us. Wait, no, never mind, do whatever you want. Okay, let's move on. If you want to.
Line 13: It all ends up with "marvel" and "curious." The speaker sticks with uncertainty until the end. Although he isn't exactly using wordplay necessarily, his strategic choice of words that mirror each other in meaning resonate with the message of the poem, which is that "not-knowing" is integral to our relationship with God, and that's not necessarily a bad thing, but it is always with us. Hence, the clever word choice of Cullen who wasn't uncertain about how to write an awesome poem.