Many of the ideas came from the Oxford Movement in the early part of the century.
Right. The only requirement for membership is a desire to stop drinking.
A drunk doesn't have to be a practicing Christian or a backslider or even a believer. The program takes care of all of it, for Protestants, Catholics, Jews, Buddhists, agnostics, atheists, whoever can grasp the concepts and makes even the most rudimentary attempts to practice them.
God is front and center - we just can't let that ever keep someone away who needs what AA has to offer, IMHO.