Why was the revelation that God has the Only Begotten Son not given in the Old Testament? Of course, we have many indications to that truth in the Old Testament, but it was only with the coming of Jesus Christ that it was spoken to humans in plain language that God has the Only-begotten Son.

How is this matter explained in the Catholicism, the Eastern Orthodoxy and main-stream Protestant Christianity?
"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:


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.


"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.

.


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)


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.  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.