For years adventist sects, including the JWs, have believed that the dead sleep at death and remain non-existent until the resurrection. Its strongest argument seems to be text in the non-escchatological book of Ecclesiastes in the Old Testament. Although a part of the canon of scripture, the book is not written by a prophet, nor is there any prophecy or recognizable doctrine contained therein. Solomon, the Son of David, had spent the last part of his life sinning against God. He was old, tired and most likely not positively disposed towards a rosey afterlife. In fact, he seems to have a poor disposition towards life and the purpose thereof. Theologians within these adventist sects argue against the rest of Christianity that Ecclesiastes teaches a doctrine that the rest of the scriptures don't seem consistent with. Instead, they argue, it was the encroachment of heretical Greek thought on a susceptible church that was opening its preaching to increasingly large numbers of gentiles.
But what of the pseudopigraphical works produced by the Jews? Or the libraries of rival Christians who were even further removed from Greek thought, such as the Nag Hammadi library, discovered in 1947 at about the same time as the Dead Sea Scrolls? Of particular interest is the Apocalypse of Abraham, a Hebrew work written from 75-150 A.D. and later translated into Slavonic. This was originally a Hebrew writing that is not produced by the Greeks, but by the Jews. Yet it not only teaches men live on after death (like most near death experiences from all cultures around the earth), but that men had pre-mortal lives:
And while [Yahweh] was yet speaking, the expanses opened, and there below me were the heavens, and I saw upon the seventh firmament upon which I stood a fire widely extended, and the light which is the treasury of life, and the dew with which God will awaken the dead, and the spirits of the departed righteous, and the spirits of those souls who have yet to be born, and judgment and righteousness, peace and blessing, and an innumerable company of angels, and the Living Ones, and the Power of the Invisible Glory that sat above the Living Ones.The Christians who wrote and kept the Egyptian library also believed in the spirits of mankind and the separation of the spirits of the just and unjust. These are considered the oldest Christian documents known -- significantly earlier than any of the scriptures in the present canon. Yet they teach man has an immortal spirit, and a number teach premortal life.
In other words, far from being Greek heresy, the doctrine that man has an eternal spirit appears to be a well-entrenched Jewish/Christian doctrine. But why doesn't the WTBTS change?