Though without more context I cannot be sure I am correct, I think
"Old Bob" refers to Robert Ingersoll, a famous/infamous "infidel"--a
U.S.-based lecturer and,I think,philanthropist-type, of the
fin-de-siecle period...He would, no doubt, be quite uncomfortable at a
prayer meeting, as his major thrust was to get people to consider the
silliness of religion....