Make plain the reason tortured Tantalus Is baited by the fickle fruit, declare
The speaker adds another paradox that God could explain: why Tantalus is tempted by "fickle" fruit.
Here's the scoop on Tantalus: he was cursed with endless hunger and every time he tried to eat a piece of fruit, the fruit tree moved out of reach. Ugh, the worst. It's torturous, because Tantalus is constantly being tantalized (get it? Tantalus, and tantalized?).
Notice our speaker calls the fruit "fickle," which sounds like a fruit that's changing its mind all the time. "Fickle" is sort of like "quibble" and "doubt" in the first two lines, so it seems like we're getting a recurring theme of uncertainty in the poem.
But wait a minute, what is Tantalus doing in a lyric poem about God? One thing we know for sure is that Tantalus is an illustration of something the speaker finds confusing. There's probably a reason why Tantalus can't get over fruit that he can never eat, but we (humans) don't really have access to that information. Bummer.
On the other hand, referencing Greek mythology lets us know the speaker is well educated. Not only is he doing cool poetry stuff like rhyming in a strict pattern, but he's name-dropping really old dudes like Tantalus. This guy must have hit the books hard back in the day.
These lines are also written in iambic pentameter, so unless this poem pulls a fast one on us and changes things up, we'll be reading a poem in strict meter, meaning each line sounds the same rhythmically.
What's the big deal? It's no accident the lines been have measured out that carefully. It may be that a strict metrical pattern is a nice balance to the list of confusions and doubts the speaker has about God.
It's like ordered chaos, or trying to make sense of something the speaker just can't quite get his head around (see Form and Meter for more about how form is like a harness the speaker uses to maneuver his way through such complicated terrain).
And Cullen's also showing off his poetic chops—a move that will become important thematically later in the poem. He does it twice over in these lines, both with the meter, and with the alliteration of "fickle fruit" and "tortured Tantalus." This guy really knows his way around a poem.
If merely brute caprice dooms Sisyphus To struggle up a never-ending stair.
First things first, a new rhyme scheme has appeared. Now we've got "Tantalus" rhyming with "Sisyphus" and "declare" rhyming with "stair." We'll call this CDCD.
This guy was a king doomed to roll a heavy boulder up a hill (or "never-ending stair," if you want to be all poetic about it) only to watch it roll back down again in the underworld. Repeat for eternity. And you thought reading poetry could be difficult!
So what's Sisyphus doing in a lyric poem? Staying on topic is this speaker's specialty so far, and Sisyphus is yet another example of something confusing that only God can understand. Futile labor? Eternity? Meaningless punishment?
Surely there's an explanation. Is it "merely brute caprice," or random cruelty that makes this happen? Only God has it (according to our speaker), and he's not telling.