Why was the revelation that God has the Only Begotten Son not given in the Old Testament?

Leibniz and Clarke debated the issue of divine freedom in the 18th century, and others continue the debate. See https://plato.stanford.edu/entries/divine-freedom/

Leibniz espoused the "Principle of Sufficient Reason":

Now, by that single principle, viz. that there ought to be a sufficient reason why things should be so, and not otherwise, one may demonstrate the being of a God, and all the other parts of metaphysics or natural theology.

Clarke argued that Leibniz' definition precluded the existence of agency. No agency, no choice, no freedom. It has been the dominant belief among Christian theologians that God is free. That means within the constraints of his nature of goodness, God can do what he wants with no higher law or philosophical necessity causing or impinging on his choice.

If God chose to withhold from Israel and the rest of humanity the sure knowledge of His trinitarian nature and the future incarnation of His Son, it was His free choice. There is no why to be answered. You can argue consistency with prior actions, with goodness and truth and promises, you can show hints and clues in prophecies, but the whole of it was God's free choice.

It is the glory of God to conceal a matter;
    to search out a matter is the glory of kings. (Proverbs 25:2)

This is not an answer to "Why?", but surely God chose thereby to magnify His glory, and to those select few who understood part of what God had concealed throug their attentive study and prayer, God surely grants glory and favor.

Revelation is... a revealing. God's revelation to humanity began in Eden, after sin entered in, and that's where the first book of the Bible, Genesis, starts (written circa 1446 B.C.). Because sin had brought both literal and spiritual death, God chose to begin revealing how his only-begotten Son would restore all things to perfection and Genesis 3:15 could be taken as the start of that revealing. That revealing culminated in the last book of the Bible, "the Revelation of Jesus Christ which God gave unto him, to show unto his servants things which must shortly come to pass" (1:1, written circa A.D. 95).

There are two considerations with regard to revealing anything. First, is whether the revealing will be instantaneous - all at once, or gradually building up a picture over time.

Second, is whether those to whom the revelation is given can see it, or whether they are totally or partially blinded. A bit of corrective eye-surgery might be needed along the way.

For a certainty, the Bible shows God gradually revealing more and more of how his only-begotten Son would eventually restore all things. The phrase "only begotten Son" only occurs in the Bible once the Son has become flesh and started dwelling among men (John 1:1-14) so that's why it seems more obscure in Bible books prior to then. It took a few thousand years for God's people to get close enough to God to start seeing the 'picture' he was revealing to them. By the time of Christ, it was as if a huge spotlight had been switched on - to him. Then, with the book of the Revelation, the Big Picture of all that was happening in the unseen heavenly realms was revealed, so that Christians could see the only-begotten Son in all his resurrection glory, defeating all the powers of darkness.

The whole of the Bible is needed to unravel this revelation, so those people who hardly bother with it are hardly going to see anything of God's revelation! Like any good "Who-done-it?" book, the crime is stated at the start (Genesis), the consequences of it shown (over centuries, and as it afflicts all humanity), clues are given as to what God's doing to bring the criminals to justice, then the one who is going to expose the criminals and deliver others appears on the scene, and then he ushers in the Day of Resurrection and Judgment. The last few chapters of the last book of the Bible expose the evil-doers for who they really are (and many of them are invisible to humans, but many humans are doing their bidding).

It's all there - for those with eyes to see. But in this sound-bite generation, which expects things instantly, and for what they want to be virtually dropped into their laps, just for the asking, they will never get it. And why should they? Their ears are closed to the gospel of Christ. To hear plain language, you cannot be deaf.

I'm answering as a Protestant who believes the entire Bible to be the inspired word of God, graciously given to fully equip God's people. By God's word we see light and the way to go, and Jesus is the Light that has come into the world, Jesus is the Way, the Truth, and the Light. Further, the whole Bible points to him - for those with eyes to see. Sadly, those who pay little attention to the Old Testament won't even see who Jesus really is in the New, for "the new is in the old concealed, and the old is in the new revealed" (so goes an old Protestant saying). Who are we to say God could have explained it differently?

"Why was the revelation that God has the [an] Only Begotten Son not given to the ancient Hebrew people?"

There are two answers to this: it was and it wasn't! Answered in reverse order:

1. It wasn't. Even though there are many hints, etc, that the coming saviour would be divine, these hints were not sufficient to control the thoughts of the Israelites/Jews in OT times concerning the coming saviour. Why is it that in the OT it is not made even more abundantly obvious that there is more than one person in the Triune Godhead and that the coming saviour would be the Son of God?

It needs to be remembered that in OT times all the surrounding nations worshipped a multiplicity of gods. A main purpose in the OT is to persuade the covenant people of God that there is ONLY ONE GOD. If the OT had spoken much about the coming Saviour being God then the temptation during the OT era would have been to suppose he would be a different god/ another god, that there is more than one God.

Once it has been established by the OT that there is only one God, the teaching of the NT about who Jesus Christ is becomes clear... he cannot be "a god" because the OT says there are no gods except the one true God.

The monotheism of the OT, then, lays the foundation stone of our understanding of the teaching of the NT concerning Christ our Lord. In the early church of the NT period, it was heavily indoctrinated Jewish believers, heavily indoctrinated in the teachings of monotheism, who tell us that Jesus is God made flesh. This lends much greater credibility to the teaching. If the Jews had not been indoctrinated in monotheism then their testimony concerning the deity of Christ could have been easily explained away... in that they were heavily indoctrinated in monotheism their testimony cannot be swept away with unbelieving rationalistic explanations.

The JW teaching that Jesus is "a god" but not "the God" is the kind of teaching which the whole thrust of the OT monotheism is designed to undermine/negate. God the Son, the Son of God, the one true God, became flesh and died for our sins. Praise Him.

1. "What think you of Christ? is the test,

To try both your state and your scheme;

You cannot be right in the rest,

Unless you think rightly of him.

As Jesus appears in your view,

As he is belovèd or not;

So God is disposèd to you,

And mercy or wrath are your lot.

.

2. Some take him a creature to be,

A man, or an angel at most;

Sure, these have not feelings like me,

Nor know themselves wretchèd and lost.

So guilty, so helpless am I,

I durst not confide in his blood,

Nor on his protection rely,

Unless I were sure he is God."

(John Newton, Pastor and former slavetrader)

2. It was. It is revealed in hints, types, and shadows that the coming saviour would be the Son of God, Divine, God made flesh. Especially noteworthy are the following OT scriptures:-

See Genesis 3:15 - the one who will defeat Satan will be a man, but it is impossible that a mere man should be able to defeat Satan, therefore he will necessarily be both Man and Divine.

I will declare the decree: the LORD has said unto me "Thou art my Son; this day have I begotten thee" Psalm 2:7

Job 19:25-26; Isaiah 7:14; Isaiah 9:6; Psalm 110 - David knew the Messiah would be one of his own descendants, yet he calls Him "my Lord", knowing He would be more than just one of his descendants; Psalm 45, the whole, esp v 6 and 7 - the Messiah will be divine; Ezekiel 44:2.