I think that this poem is kind of a contradiction because Cullen starts out by saying that he doesn't doubt God is good, but then he makes a list of certain events that say otherwise. He brings up events like that of Tantalus and Sisyphus which were Greek characters that got eternal punishments. With these, he maybe was trying to say that how did God allow the punishment of these people and why didn't he do anything to stop it? In the third to the last line, he says "What awful brain compels His awful hand." and with this said he kind of makes the statement that how could God have the mind that he has and also the power that lies in his hands.
Cullen also mentions at the end that God made him both black and a poet which to him where bad things. This could be seen as him saying that he still felt that being colored was bad and that it was even worse because people didn't like when they spoke their mind in poetry. This really shows how alienation was very big during this time period, which must have been really tough for many rising poets.