The concept of the soul's supposed immortality was first taught in ancient Egypt and Babylon. "The belief that the soul continues in existence after the dissolution of the body is...speculation...nowhere expressly taught in Holy Scripture...The belief in the immortality of the soul came to the Jews from contact with Greek thought and chiefly through the philosophy of Plato, its principal exponent, who was led to it through Orphic and Eleusinian mysteries in which Babylonian and Egyptian views were strangely blended" (Jewish Encyclopedia, 1941, Vol. 6, "Immortality of the Soul," pp. 564, 566).
Plato (428-348 B.C.), the Greek philosopher and student of Socrates, taught that the body and the "immortal soul" separate at death. The International Standard Bible Encyclopedia comments on ancient Israel's view of the soul: "We are influenced always more or less by the Greek, Platonic idea that the body dies, yet the soul is immortal. Such an idea is utterly contrary to the Israelite consciousness and is nowhere found in the Old Testament" (1960, Vol. 2, p. 812, "Death").
Early Christianity was influenced and corrupted by Greek philosophies as it spread through the Greek and Roman world. By A.D. 200 the doctrine of the immortality of the soul became a controversy among Christian believers.
The Evangelical Dictionary of Theology notes that Origen, an early and influential Catholic theologian, was influenced by Greek thinkers: "Speculation about the soul in the subapostolic church was heavily influenced by Greek philosophy. This is seen in Origen's acceptance of Plato's doctrine of the preexistence of the soul as pure mind (nous) originally, which, by reason of its fall from God, cooled down to soul (psyche) when it lost its participation in the divine fire by looking earthward" (1992, "Soul," p. 1037).
Secular history reveals that the concept of the immortality of the soul is an ancient belief embraced by many pagan religions. But it's not a biblical teaching and is not found in either the Old or New Testaments.