The ancient Hebrews did not possess an understanding of “immortality of the soul” as we would know it today. Their conception of the afterlife was an afterthought. The emphasis in the Torah is on this world and on faithful adherence to the commandments enshrined in the Mosaic Covenant, for oneself and one’s progeny. The limited references to the afterlife and to practices concerning it do, nevertheless, indicate quite clearly that the ancient Jews believed that “something” of people lived on after death in a shadowy state of existence known variously as Sheol or Abbadon. They dwelled in this place as “shades”, that is wraith-like, ethereal and limpid “remnants” of their human self: almost like shadows. This is the earliest afterlife view attested to in the Hebrew Bible. It was not believed that the “shades” could rise up or “free” themselves. They were without knowledge, wisdom, love, hate or anything else one would associate with life. Nonetheless, they were capable of being summoned as Samuel’s “shade” was by Saul in the witch of Endor incident. Shades were therefore conscious.
Gradually, a new belief began to develop that there was a differentiation between the state of the blessed dead and the unholy dead. The “blessed” arrived in “Abraham’s Bosom” within Sheol. A limited belief in judgement according to works thus grew.
This worked in tandem with a growing belief in “resurrection from the dead” in a bodily sense, which we can see quite evidently in the Book of Daniel, under Zoroastrian influence apparently during the period of the Babylonian exile.
The doctrine of the “immortality of the soul” is thus implicit in ancient Israel.
It arose, actually, in ancient Greece. Under Hellinistic influences the Jews slowly came to believe in a hell and in immortality of the soul. The deuterocanonical book of Wisdom, which for Catholics is sacred writ, attests to the soul’s immortality and was written under the influence of Greek philosophy.
In Book X of Plato’s"The Republic" (c. 380 BCE), there is an account of the afterlife as the great philosopher views it, known as ‘The Myth of Er’. This is the foundational text of Western philosophy.
Plato’s Myth of Er greatly influenced subsequent religious and philosophical thought, up to and including our very idea of heaven and hell.
Er was slain in battle but came back to life 12 days later to tell the living of that which he had seen. During these 12 days, his soul went on a journey to a meadow with four openings, two into the heavens above and two into the earth below.
Judges sat in this meadow and ordered the good souls up through one of the openings into the heavens and the bad ones down through one of the openings into the earth. Meanwhile, clean and bright souls floated down to the meadow from the other opening into the heavens, and dusty and worn out souls rose up to the meadow from the other opening into the earth.
Each soul had returned from a thousand year journey, but whereas the clean and bright souls spoke merrily of that which they had enjoyed in the heavens, the dusty and worn out souls wept at that which they had endured in the underground. Souls that had committed heinous crimes, such as those of tyrants or murderers, were not permitted to rise up into the meadow, and were condemned to an eternity in the underground
Plato also believed in the transmigration of souls however.
In his Phaedo Plato emphasises that the some souls “never return” to reincarnate again but are cast into Tartaros (Hell) where they return “no more”.
He believed that cycles of birth and re-birth for others could be seemingly endless, however.
Alan E. Bernstein notes therefore in his book, “The Formation of Hell”:
“…Plato makes the same argument in his Phaedo. A soul that dies cannot pay for the evil it has wrought, and so the mortality of the soul would be a boon to the wicked, who would escape with lighter punishments than they deserve. Justice, therefore, demands the immortality of the soul; and the immortality of the soul makes eternal punishment possible. It seems, then, that Plato is the earliest author to state categorically that the fate of the extremely wicked is eternal punishment…”
The Formation of Hell: Death and Retribution in the Ancient and Early … - Alan E. Bernstein - Google Books
Interestingly Plato also came to the awareness in his Timaeus of a single divine creator.