My basic theory that the ancient Greek philosophers did not really have a direct role in seeing belief and reason as different, least of all, as opposites, has been verified.
Philosophers from the earliest times in Greece tried to distill metaphysical issues out of these mythological claims. Once these principles were located and excised, these philosophers purified them from the esoteric speculation and superstition of their religious origins. They also decried the proclivities to gnosticism and elitism found in the religious culture whence the religious myths developed. None of these philosophers, however, was particularly interested in the issue of willed assent to or faith in these religious beliefs as such (From the IEP; emphasis added).The second assumption is that the elevation of one against the other happened much before the Enlightenment. Much before Descartes arrives on the scene, belief and reason are arch-rivals. The earliest I can think of, to whom I can 'credit' the juxtaposing of one as the rival of the other, is St Paul. But is he? Who else? and most importantly why?