Notice how God has to "stoop" here? The speaker doesn't come out and say anything about weakness, but it's clear that God has dominion over us and we're too powerless to understand why God does what He does. In fact, the speaker doesn't really have an answer at this point either; he just says, God could tell us if He wanted to, but He doesn't. Sounds like we can't do much but accept whatever God gives us.
The little buried mold continues blind (3)
We've already mentioned this line as an example of suffering but it's also a form of weakness. The mold is too weak to change his plight in life. God made the mole how he is, and there's not a whole lot the mole can do about it. It is what it is, right?
Why flesh that that mirrors Him must some day die, (4)
Here's another example of weakness. In short, humans are powerless over death. There's not a whole lot we can do about that, and the speaker is using this as another example of how we're powerless over God's design of life. That is unless you find the fountain of eternal life, in which case, call Shmoop ASAP and let us know where it is, thanks.
Make plain the reason tortured Tantalus
Is baited by the fickle fruit, declare
If merely brute caprice dooms Sisyphus
To struggle up a never-ending stair. (5-8)
We're bunching these two examples of Greek mythology together because they're similar examples of how the speaker is giving us a more abstract example of weakness. Both Tantalus and Sisyphus are powerless to change their situations. No matter how hard he tries, Tantalus can't get any food to end his hunger, and Sisyphus can't finish his endless task of rolling a boulder uphill. Of course, Sisyphus might look like Arnold Schwarzeneger after an eternity of such grueling exercise, but we're not entirely sure that's a good thing.
Inscrutable His ways are, and immune To catechism by a mind too strewn With petty cares to slightly understand What awful brain compels His awful hand. (9-12)
Here the speaker comes right out and says that our minds just aren't strong enough to understand God's ways. God is "immune" to our "petty cares". In other words, we just don't have the brain power to grasp something like God's reasons for the way things are. Little brains, all of us! Well, the speaker's not saying that exactly, but he is saying that our intellectual faculties only get us so far in understanding life.