An English joke that I can't understand.

First of all, it must be pointed out that this joke is quite racist. The joke is to satirize the poor English of black people, because in the past, black people had a low level of education, and there were many grammatical irregularities in language use, such as "double negation" that I am who I am and I don't have money. The white priest above means that God didn't make such grammatical mistakes, so God is white.