Fatuous: adjective: complacently or inanely foolish

Source: https://www.merriam-webster.com/dictionary/fatuous

From: “And this, Helga decided, was what ailed the whole Negro race in America, this fatuous belief in the white man’s God, this childlike trust in full compensation for all woes and privations in “kingdom come.”Sary Jones’s absolute conviction, “In de nex’ worl’ we’sall recompnse’,” came back to her.”

The word fatuous is used to describe how blindly the black people that Helga associates with, follow Christianity.