In chapter 24 of “To Kill a Mockingbird,” how is Christianity a validation of prejudice, and why are the conversations that are had by the missionary circle members considered pure hypocrisy?

In chapter 24, Scout is made to participate in Aunt Alexandra's missionary circle. One of the topics of discussion is the Mrunas, a tribe in Africa:

Mrs. Merriweather’s large brown eyes always filled with tears when she considered the oppressed. “Living in that jungle with nobody but J. Grimes Everett,” she said. “Not a white person’ll go near ‘em but that saintly J. Grimes Everett.”

Mrs. Merriweather has presented J. Grimes Everett as a hero simply for interacting with the Mrunas. Her opinions are based on contemporary societal belief that groups of people that have not been Christianized are less advanced and even primitive in comparison to Christianized society.
To be prejudiced is to have preconcieved notions of an individual or group of people that are not based on actual experience or reason. In this case, it is a biased form of conceptualizing others by means of religion. Mrs. Merriweather genuinely believes that J. Grimes Everett and Christianity are saving the Mrunas by introducing them to the true way of life and faith.
Mrs. Meriweather is lamenting over the Mrunas and their supposedly poor and faithless way of life without demonstrating sympathy for Maycomb's struggling black population. Within the social circle, the ladies praise J. Grimes Everett for his missionary work in Africa, while gossiping about those who support equality of black people in the context of Tom Robinson's trial.
Through the missionary circle, Harper Lee accentuates prejudice by way of irony. Mrs. Merriweather acts on the pretense of piousness and concern for the Mrunas, but, in reality, she is an ignorant and prejudiced person.