That of the Old Testament, along with what are historically the daughter faiths, Christianity and Islam.
Moslems and Jews were applying Aristotelian philosophy to rigorously monotheistic faiths; Christianity had been encouraged by Platonism in teaching a trinity of divine persons, and Platonism of a certain order long dominated the middle ages as part of the Augustinian tradition.
Modified though never essentially changed, (1) by contact with the star-worship of the Chaldaeans, who identified Mithras with Shamash, god of the sun,(2) by the indigenous Armenian religion and other local Asiatic faiths and (3) by the Greeks of Asia Minor, who identified Mithras with Helios, and contributed to the success of his cult by equipping it for the first time with artistic representations (the famous Mithras relief originated in the Pergamene school towards the 2nd century B.C.), Mithraism was first transmitted to the Roman world during the 1st century B.C. by the Cilician pirates captured by Pompey.
The census of 1890 divided the population into 14,179,615 Roman Catholics, 1 43,743 Protestants, 3300 of all other faiths, 7257 of no religious profession, and 600,000 unchristianized Indians.
Christianity was essentially a proselytizing religion, not content to appeal simply to one class or race of people, and to be one among many faiths, but believing in the falsity or insufficiency of all others and eager to convert the whole world.