Thus the sojourn of the soul in the world below for the thousand years which must elapse before it could be born again, was a period of cleansing from ancient sin. This idea of purification we have already seen to be as old as the Orphics; it was made an important element by Plato; and indeed all who held to the doctrine of rebirths regarded the periods between earthly existences as times of moral punishment and cleansing. There were certain analogies in Mithraism. Orthodox Christianity could not adopt the doctrine of metempsychosis, although some Gnostics found this possible, by rejecting the resurrection of the body. But beyond question the Greek doctrine of post-mortem purgation from sin, combining with ideas inherited from the Old Testament, has been influential in the development of a Christian belief in purification, especially by fire, in an intermediate state between death and paradise. The doctrine of purgatory, in somewhat different forms, has been held by both the Eastern and the Western Churches. Although this doctrine did not become a definite part of the theology of the Western Church until the time of Gregory the Great (590-604), nevertheless traces of it can be found in the earlier Church writers. Origen held that even the perfect must pass through fire after death;[39] St. Augustine was less confident, but he thought it not past belief that imperfect souls might be saved by cleansing flames.[40] The Western Church, from St. Thomas Aquinas in the thirteenth to Bellarmino in the sixteenth century held the doctrine that the cleansing fire was as material as that of any Stoic; but today that view has in large part been abandoned.[41] These two illustrations must suffice to suggest the ways in which Christian thought was influenced by its pagan environment.