Although the speaker gives God kudos that He could explain everything about life, the poem also revolves around the speaker's awareness of unanswerable paradoxes in life. At least, unanswerable to us small-brained humans. This paradox continues throughout the poem and the speaker eventually uses his lack of understanding as a source of wonderment when he says he marvels that God makes "a poet black."
Line 3: The blind mole is the first apparent paradox in the poem. If God is good, why does he doom a mole to live in never-ending darkness? On the other hand, the poet says the mole "continues blind." So, maybe it seems like the mole has a bad deal, but he's actually able to continue despite his condition in life. This line can be read both ways, and that's what keeps the sonnet interesting over time.
Line 4: Okay, if God's so good, why does His creation, specifically human beings, have to die? The speaker uses a bit of metonymy here, and says "flesh that mirrors Him," meaning humans. If God made us, why does he eventually end us with death? However, the speaker is speaking out of a Christian conception of God, which declares death is temporary. So we could argue either way: this line is about a meaningless end to life or it suggests something we don't understand about life but which God eventually answers for us. Confusing, huh? That's what the speaker thinks, too.
Lines 5-6: The speaker goes Greek here and name drops Tantalus, a man doomed to eternal hunger which can never be satisfied. In fact, every time he reaches for a piece of fruit, the "fickle fruit" moves just out of reach. That sort of wordplay is a figurative personification of fruit as something that can change its mind, like someone taunting you with a bite of food when you're starving. Who on earth would wish that on anyone? Our speaker's a thoughtful guy, but even he doesn't have the answer. Just another question to put on the checklist of how life is confusing.
Lines 7-8: Staying with the Greek characters, the speaker mentions Sisyphus. That guy is certainly in a pickle. Not only does he have to roll a massive rock up a hill, but he has to do it up a "never-ending stair," which is the imagery the speaker uses to evoke eternal struggle. Never-ending labor? Struggle? Meaninglessness? Certainly, a good God wouldn't make life like that, yet Sisyphus is a myth that's endured the test of time as a symbol of struggles in life. What's the point, right? Well, our speaker says there is one, but we just don't know what it is. Keep on rollin'.
Lines 13-14: The speaker is marveling at God's goodness here. The paradox here may be a bit ironic, or tongue-in-cheek. The speaker is sincerely celebrating his identity as a black poet, but also talking about what a struggle it is. A marvelous struggle, but a struggle nonetheless.