Why do I think ancient people adopted the belief in an immortal soul? — The likely answer is that the ancients and medieval people were generally indoctrinated to believe the dogma of an immortal soul, along with other dogma about the fate of the soul after death. In the Christian period, when they were not so indoctrinated or resisted the indoctrination, they were terrorized into believing (or at least outward assent to the belief).