God set eternity in man’s heart and in longing for that which is unknowable, the inspired men of the Old Testament revealed glimpses from God of that home. And truth to tell, though we have more figures of speech, it is hard to say that we understand much more than they did (Eccl 3:11).
But as for me I know that my Redeemer lives, And at last he will stand up upon the earth: And after my skin, [even] this [body], is destroyed, Then without my flesh shall I see God; Whom I, even I, shall see, on my side, And mine eyes shall behold, and not as a stranger. My heart is consumed within me. (Job 19:26). The ASV translation, “without my flesh” is justified by “And after my skin, this, is destroyed.” Clearly, after his life was over, Job expected to see God.
Peter quotes Psa 16:10-11 and applies it to the resurrection of the Christ. As with most Messianic passages, it also had a meaning for the time written. Regardless, it expresses absolute faith in an undying soul that comes forth from the grave, from the realm of the dead.
Arise, O Jehovah, Confront him, cast him down: Deliver my soul from the wicked by thy sword; From men by thy hand, O Jehovah, From men of the world, whose portion IS IN [THIS] LIFE,… As for me, I shall behold thy face in righteousness; I shall be satisfied, WHEN I AWAKE, WITH BEHOLDING THY FORM. Men of the world have their portion in this life; The Old Testament righteous lived in hope of a resurrection where they would be with God and see him.
Asaph nearly slipped when he considered the rich for “they have more than heart could wish” and “there are no pangs in their death.” But then he “went into the sanctuary of God and considered their latter end.” (Psa 73:4,7,17). What latter end if there are no pangs in their physical death and there is no eternity? Certainly this psalmist believed in after-life punishment of the wicked. And, he believed in a reward in heaven for the righteous, “Whom have I in heaven [but thee]? And there is none upon earth that I desire besides thee. My flesh and my heart fail; [But] God is the strength of my heart and my portion for ever.” (24-26).
For he shall see it. Wise men die; The fool and the brutish alike perish, And leave their wealth to others. They are appointed as a flock for Sheol; Death shall be their shepherd; And the upright shall have dominion over them in the morning; And their beauty shall be for Sheol to consume, That there be no habitation for it. But God will redeem my soul from the power of Sheol; For he will receive me. (Psa 49:10,14-15). Shall we sing a verse of “In The Morning of Joy?” The righteous will be received, redeemed from the power of death and have dominion. YES THEY KNEW!
Dan 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt. It is not clear in the context that this concerns the final resurrection. Many think it does.
Before Jesus came revealing more, the Pharisees studied the Old Testament and determined that it taught eternal life, eternal reward and eternal judgment. The Sadducees were worldly and could only come up with silly riddles to mock that belief—sort of like the riddle, “What would happen to a man on the way to baptized and a tree fell on him?” These establish nothing. Jesus himself supported the Pharisees, saying, “The scribes and the Pharisees sit on Moses' seat: all things therefore whatsoever they bid you, [these] do and observe: but do not ye after their works; for they say, and do not.” (Mt 23:1ff).
The rich young ruler asked Jesus, "What must I do to inherit Eternal Life?" If the Old Testament does not teach eternal life as a reward, where did he get his notions about it? When you finish considering that, think about the Lawyer who asked the same question in Lk 10:25. A Lawyer had no interest other than the Torah, the five books of Moses.
There are many more such passages in the Old Testament. Some may be less clear, some may speak of eternal life and again, may not. However, once we have established as clearly as has been done above that they knew these things, it does no violence to truth to understand the doubtful ones as supporting belief in eternal life.
In the resurrection morning,
When the trump of God shall sound,
We shall rise, we shall rise
Then the saints will come rejoicing,
And no tears will e'er be found
We shall rise, we shall rise.