It’s true that the doctrines of salvation and the eternal afterlife (both for the saved and the damned) developed quite a bit in the inter-testamental period, and then all the more so in the New Testament. But there were also many clear signs or precursors of it (some remarkably explicit) all along. The book of Job seems to allude to an eternal consciousness in a resurrected body in some sort of paradisal state with God:
Job 19:25-27 (RSV) For I know that my Redeemer lives, and at last he will stand upon the earth;  and after my skin has been thus destroyed, then from my flesh I shall see God,  whom I shall see on my side, and my eyes shall behold, and not another. (cf. 14:12-15)
The prophet Isaiah (8th c. BC) taught similarly:
Isaiah 26:19a Thy dead shall live, their bodies shall rise. O dwellers in the dust, awake and sing for joy! . . .
This is hardly a shadowy temporary existence in Sheol and then annihilation. But there is much more, too:
Genesis 5:24 Enoch walked with God; and he was not, for God took him.
2 Kings 2:11 And as they still went on and talked, behold, a chariot of fire and horses of fire separated the two of them. And Eli’jah went up by a whirlwind into heaven.
Psalms 16:10-11 For thou dost not give me up to Sheol, or let thy godly one see the Pit.  Thou dost show me the path of life; in thy presence there is fulness of joy, in thy right hand are pleasures for evermore.
Psalms 21:4, 6 He asked life of thee; thou gavest it to him, length of days for ever and ever. . . .  Yea, thou dost make him most blessed for ever; thou dost make him glad with the joy of thy presence.
Psalms 23:6b . . . I shall dwell in the house of the LORD for ever.
Psalms 49:7-9, 15 Truly no man can ransom himself, or give to God the price of his life,  for the ransom of his life is costly, and can never suffice,  that he should continue to live on for ever, and never see the Pit. . . .  But God will ransom my soul from the power of Sheol, for he will receive me. . . .
Psalms 73:23-26 Nevertheless I am continually with thee; thou dost hold my right hand.  Thou dost guide me with thy counsel, and afterward thou wilt receive me to glory.  Whom have I in heaven but thee? And there is nothing upon earth that I desire besides thee.  My flesh and my heart may fail, but God is the strength of my heart and my portion for ever.
Daniel 7:18 But the saints of the Most High shall receive the kingdom, and possess the kingdom for ever, for ever and ever.
Daniel 12:1-3 “At that time shall arise Michael, the great prince who has charge of your people. And there shall be a time of trouble, such as never has been since there was a nation till that time; but at that time your people shall be delivered, every one whose name shall be found written in the book.  And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.  And those who are wise shall shine like the brightness of the firmament; and those who turn many to righteousness, like the stars for ever and ever.
The notion of spiritual salvation is present in the Old Testament as well:
Psalms 34:4-8 I sought the LORD, and he answered me, and delivered me from all my fears.  Look to him, and be radiant; so your faces shall never be ashamed.  This poor man cried, and the LORD heard him, and saved him out of all his troubles.  The angel of the LORD encamps around those who fear him, and delivers them.  O taste and see that the LORD is good! Happy is the man who takes refuge in him!
Psalms 51:1-14, 17 Have mercy on me, O God, according to thy steadfast love; according to thy abundant mercy blot out my transgressions.  Wash me thoroughly from my iniquity, and cleanse me from my sin!  For I know my transgressions, and my sin is ever before me.  Against thee, thee only, have I sinned, and done that which is evil in thy sight, so that thou art justified in thy sentence and blameless in thy judgment.  Behold, I was brought forth in iniquity, and in sin did my mother conceive me.  Behold, thou desirest truth in the inward being; therefore teach me wisdom in my secret heart.  Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.  Fill me with joy and gladness; let the bones which thou hast broken rejoice.  Hide thy face from my sins, and blot out all my iniquities.  Create in me a clean heart, O God, and put a new and right spirit within me.  Cast me not away from thy presence, and take not thy holy Spirit from me.  Restore to me the joy of thy salvation, and uphold me with a willing spirit.  Then I will teach transgressors thy ways, and sinners will return to thee.  Deliver me from bloodguiltiness, O God, thou God of my salvation, and my tongue will sing aloud of thy deliverance. . . .  The sacrifice acceptable to God is a broken spirit; a broken and contrite heart, O God, thou wilt not despise.Proverbs 15:24 The wise man’s path leads upward to life, that he may avoid Sheol beneath.
Isaiah 45:17, 21-22 But Israel is saved by the LORD with everlasting salvation; you shall not be put to shame or confounded to all eternity. . . .  . . . And there is no other god besides me, a righteous God and a Savior; there is none besides me.  “Turn to me and be saved, all the ends of the earth! For I am God, and there is no other.
Jeremiah 17:14 Heal me, O LORD, and I shall be healed; save me, and I shall be saved; for thou art my praise.
Jeremiah 31:33-34 But this is the covenant which I will make with the house of Israel after those days, says the LORD: I will put my law within them, and I will write it upon their hearts; and I will be their God, and they shall be my people.  And no longer shall each man teach his neighbor and each his brother, saying, `Know the LORD,’ for they shall all know me, from the least of them to the greatest, says the LORD; for I will forgive their iniquity, and I will remember their sin no more.”
[see also the many instances of “salvation” in the Old Testament. Most likely refer to “physical salvation” from enemies and death, but some may have a double application, and some may refer to spiritual salvation only]
The idea of being abandoned to Sheol is similar to eternal hellfire (a developmental precursor or “kernel” of it). Thus, eternal life or eschatological salvation is described as “thou dost not give me up to Sheol” (Ps 16:10). God delivers or rescues the righteous from Sheol (“he brings down to Sheol and raises up”: 1 Sam 2:6; cf. Ps 30:3; 49:15; 86:13; 89:48):
Deuteronomy 32:22a For a fire is kindled by my anger, and it burns to the depths of Sheol, . . .
Psalms 6:5 For in death there is no remembrance of thee; in Sheol who can give thee praise?
Psalms 9:17 The wicked shall depart to Sheol, all the nations that forget God.
Psalms 31:17b . . . let the wicked be put to shame, let them go dumbfounded to Sheol.
Isaiah 14:11 Your pomp is brought down to Sheol, the sound of your harps; maggots are the bed beneath you, and worms are your covering. (cf. 14:15 and Jesus’ reference to worms in hell: “where their worm does not die, and the fire is not quenched”: Mk 9:48)
Isaiah 38:18b . . . those who go down to the pit cannot hope for thy faithfulness.
Isaiah 66:24 “And they shall go forth and look on the dead bodies of the men that have rebelled against me; for their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh.”
Jeremiah 15:14b . . . in my anger a fire is kindled which shall burn for ever. (cf. 17:4)
Daniel 12:2 And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
[see also many references to “the pit”: equivalent to Sheol]
Photo credit: Transfiguration of Jesus [he appeared with Elijah and Moses], by Carl Bloch (1834-1890) [public domain / Wikimedia Commons]