Modern conceptions of the supernatural and the soul have developed over many centuries with a complex array of influences. One major influence on today's concept of the 'soul' seems to have come from the Gentile tradition, which in turn came from ancient Greek thought. Socrates had his own ideas on the soul and his original arguments may have had a large impact on the concept of the soul in modern Christianity.
In an essay I've just completed, I go back and take a look at the arguments Socrates used to justify the existence of a distinct soul...
Simmias' Harmony: Socrates' Arguments for A Distinct Soul Dissected
(note: Sorry for the late posting of this - it took me longer to complete than I had anticipated)