Why doesn't anyone listen to Reverend Jameson on page 53 of A Gathering of Old Men? Why do you think Gaines chooses to make the minister an ineffectual character, perhaps one of the weakest we encounter?

No one will listen to Reverend Jameson as he cautions acquiescence in the face of racial oppression. Far from doing so out of deeply-held religious convictions, this supposed man of God is simply acting out of fear and self-preservation. The old men don't respect him because they resent him for his cowardice and lack of firm resolve. He clearly isn't on their side, and is prepared to allow a blatant injustice to take place in his parish.
One would've thought that Jameson would stand with his community in the challenge they face. But no, he's thinking only of himself, and the members of his flock will never forgive him for it.
Organized religion has often been used throughout history to legitimize all manner of social evils. And the Reverend Jameson's timidity and lack of spiritual leadership is part of this long, ignoble tradition. His moral inertia bolsters the prevailing system of racial oppression, whether he intends this or not. In making a man of the cloth such a yellow-belly, Gaines is drawing attention to the impotence of formal religion in situations of acute tension, where resolute action, sometimes involving violence, is an absolute necessity.

The situation you are referring to is likely when Beulah tells Reverend Jameson to go home and Rooster asks Dirty Red if he should shoot Reverend Jameson. As the question points out, no one in the community respects the reverend, even though he is a clergyman. The reason that people don't respect him is that he doesn't want the African-American community to stand up for themselves against white plantation owners such as Fix Bouton. Instead, Jameson is afraid whites will burn his parish down if his community starts to protest. He would rather meekly accept the status quo, even if it means that African-Americans are constantly harassed and abused. Perhaps by making Jameson weak, Gaines is suggesting that religion as it is usually practiced has not helped African-Americans in their quest for justice and equality with whites. Instead, the African-Americans in the novel must turn to community solidarity to gain justice.