An email correspondent asked this:
Would I be correct in assuming your answer is in essence that you haven't a clue, guess, thought, speculation, or hypothesis concerning why God postponed telling man anything significant about the Trinity, Heaven and Hell until two-thirds of the generations of man were dead?
I had earlier answered a less-developed question, but he has pressed, desiring a fuller explanation. I thought my answer would be worth sharing here (but I could have been wrong to think that):
No, it's not that I don't have a clue, it's just that God almost certainly hasn't revealed all His reasons.
So I'll take another stab at it.
1st, He may have waited until 2/3 of generations of man were dead, but certainly not until 2/3 of all humans that will ever live have died. Also, we have no idea how many generations of man WILL pass by before the end comes. This earth could well go on so long that God's delay in fuller self-revelation would leave an inconsequential segment of humanity unrevealed-to in the past, if you will, compared to how many will have lived since.
2nd, getting to what He has explicitly said, like I said, He waited until the time was full. The only "why" I can give is that God ordains all that comes to pass, so He planned that Christ would come when He did. It's all according to His plan.
3rd, God desired to glorify Himself and provide evidence to humanity of His existence, goodness, and provision by giving prophecies thru His prophets and Scripture beforehand and then carry out and fulfill said prophecies.
4th, related to #3, God apparently enjoys foreshadowing the future and preparing types of the still-to-come antitypes/fulfillments. An example is Adam (type/foreshadowing) and Jesus Christ (2nd Adam, as fulfillment, antitype) (see Romans 4-5).
5th, God is glorified thru His people exercising faith in His goodness and provision based on what He has done before and His promises. The books of Hebrews (chapter 11) and 1 Peter discuss how the holy men and women who came before us were recipients of promises, but in most cases, they did not receive the promised-for things b/c they were for the benefit of future generations. They were mostly to be fulfilled in Jesus Christ.
And I reiterate that the OT has a doctrine of afterlife, Hell and Heaven both are foreshadowed in significant ways.
The Triune nature of God is foreshadowed in, among other ways, multiple OT references to the Holy Spirit and angelophanies of the "angel of the LORD", who is clearly divine and whose identity is, in fact, very probably the preincarnate Christ, and the oft-stated axiom that "to see God is to die". Yet people saw God. How, if not by a Triune God?