Before going blind, the speaker has high hopes for what he might accomplish in the future. He says he would have been a supremely useful servant of God. But we can't know if his motives are truly selfless, or if he is an ambitious guy who now struggles to come to terms with a personal upheaval. As he looks to the future, he compares his situation to the third servant from the New Testament "Parable of the Talents" in Matthew 25. Because he has not increased his master's wealth, this servant is cast into the darkness. Considering that the speaker already feels he lives in the darkness, what further punishment does he expect? At the end of the poem, patience gives him a new plan: he should wait until God calls on him to serve.
The poem upholds the Protestant idea that worldly labor is necessary for the benefit of the individual soul and not for the realization of God's kingdom on earth.
The speaker believes he has nothing more to fear from God except "chiding," because he has already been cast into "darkness."