After a discussion on another thread I've decided to make this one. There's pretty much 0 chance of this not becoming a debate so this section it is in.
I would like Christians to tell me how they know what to look for in the Messiah, what he is supposed to do and so on, using only the Tanakh ('OT').
Go!
Greets. I was just about to take a nap when I logged on and saw this question. Thought I would share some thoughts. Firstly, I respect the Jews. I lived for a long time in a large community of Jews in Glendale, Colorado. Had several neighbors that were Jews. In fact I would babysit my neighbor's son after school until she got off of work and they were both Jews. And I have had many stimulating conversations with Jews. The orthodox Jew and a Jehovah's Witness have a lot in common. I think it is improper to disrespect another person's beliefs. And when I can use what a person believes in to help persuade them to understand something I try.
Even Paul said 'I became all things to all people that I might somehow save some.' He was a Jew. And when he was around Jews he still performed the Jewish customs so as not to offend a Jew. When the Sanhedrin was trying to convict him to death he used a rift that was between the Jewish sects of the Sadducees (who did not believe in the resurrection) and the Pharisees. Paul said he was being condemned on the fact that he believed in the resurrection. That got a lot of Pharisees to actually take his side.
The very first prophecy I can think of referring to the Messiah is in Genesis 3:15. Talking to the serpent Jehovah God said:
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And I will put enmity between you and the woman and between your offspring and her offspring. He will crush your head, and you will strike him in the heel.”-Genesis 3:15.
The serpent was not the source of the deception against Eve. So God was talking to the spirit that used the serpent to lie to Eve. He said that there would be two offspring. One of the serpent, and one of the woman. This Messiah, the seed, or offspring, would crush the head of the serpent.
After the flood Jehovah turns his attention to his faithful servant Abram and turns his name to Abraham (meaning father of a crowd or many). And he said this about the offspring (singular, not plural, that is to one person, the Messiah that was to come, and not all his descendants):
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And by means of your offspring all nations of the earth will obtain a blessing for themselves because you have listened to my voice.’”-Genesis 22:18.
So by means of the Messiah all the nations of the earth would be blessed. Everyone would be blessed by means of him. Not just the Jews.
Jehovah God made his covenant with Abraham on Nisan 14, 1943 B. C. E. 430 years later Moses lead the Israelites out of slavery to the Egyptians on Nisan 14, 1513 B. C. E. Just as was foretold that they would be there for 400 years:
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Then He said to Aʹbram: “Know for certain that your offspring will be foreigners in a land not theirs and that the people there will enslave them and afflict them for 400 years."-Genesis 15:13.
Later God reaffirmed that covenant with Abraham's son Isaac, the son through Sarah, and not his concubine Hagar (who birthed Abraham his firstborn Ishmael). Abraham also had 6 other sons with a concubine after Sarah died. -Genesis 25:1, 2. One of them was Midian. Later a priest of Midian would give his daughter Zapaorrah to the Messiah, Moses.
Jacob's name was turned to Israel, and on his deathbed he foretold that Messiah would come through the lineage of his forth son, Judah:
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The scepter will not depart from Judah, neither the commander’s staff from between his feet, until Shiʹloh comes, and to him the obedience of the peoples will belong."-Genesis 49:10.
Here we are told that peoples (plural not just the Jews) would serve the Messiah. That is he would be king over them. The commander's staff and the scepter both represent authority and power to rule.
Almost 2,000 years after God made the covenant with Abraham, an everlasting promise that would never be broken, he continued to reveal his purpose concerning the covenant to Abraham, and made the covenant with King David:
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I will raise up your offspring after you, your own son, and I will firmly establish his kingdom. He is the one who will build a house for my name, and I will firmly establish the throne of his kingdom forever."-2 Samuel 7:12, 13.
The Messiah would thus be king and have an everlasting rulership.
That the Messiah was not only to rule as king but also as priest is seen in the prophecy of Psalm 110:
Jehovah declared to my Lord:
“Sit at my right hand
Until I place your enemies as a stool for your feet.”
Jehovah will extend the scepter of your power out of Zion, saying:
“Go subduing in the midst of your enemies.”
Your people will offer themselves willingly on the day of your military force.
In splendid holiness, from the womb of the dawn,
You have your company of young men just like dewdrops.
Jehovah has sworn an oath, and he will not change his mind:
“You are a priest forever
In the manner of Mel·chizʹe·dek!”
-Psalm 110:1-4.
This actually tells us that the Messiah would rule in the midst of his enemies and subdue them. And it has been sworn, an oath, by God himself that he is a priest, "In the manner of Melchizedek."
How could a priest come out of the tribe of Judah if the tribe of Levi had the priesthood? This was not a priesthood based on the Mosaic law covenant given to the Israelites. We are told by Jehovah God in this prophecy that is not the case. It is a priesthood in the manner of Melchizedek.
Now Melchizedek was the king and priest of Salem before any offspring came out of Abraham's loins. And it was the priest Melchizedek who blessed Abraham, and by extension all of his descendants. And the one who blesses is greater than the one being blessed. So the priesthood and kingship the Messiah would attain is of far superior and lasting value than that of the priesthood of Levi and Aaron.
The Isarelites weren't even supposed to have a human ruler under the Mosaic law. It was their insistence on having one that God finally relinquished and had Samuel anoint Saul of the tribe of Benjamin as king, and then later David of the tribe of Judah. Among others.
So we have seen that the Messiah would crush and end the rebellion that started in the garden of Eden, by giving a head wound to the serpent. He would come in the line of Judah, he would command the subjugation of many peoples, not just the Jews. That all peoples of the earth would be blessed by means of him. That he would have an everlasting kingdom and would rule as king and priest forever.
That he was God's son and that God would give his son not only the rulership over Isreal but over the entire earth is told us in prophecy in Psalm 2:
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Let me proclaim the decree of Jehovah;
He said to me: “You are my son;
Today I have become your father.
Ask of me, and I will give nations as your inheritance
And the ends of the earth as your possession."
-Psalm 2:7, 8.
So the Messiah receives the entire earth as his inheritance. Jehovah God gives it to him to rule.
Daniel forsees the coronation of the Messiah. And it is not done on earth, but heaven itself:
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I kept watching in the visions of the night, and look! with the clouds of the heavens, someone like a son of man was coming; and he gained access to the Ancient of Days, and they brought him up close before that One. 14 And to him there were given rulership, honor, and a kingdom, that the peoples, nations, and language groups should all serve him. His rulership is an everlasting rulership that will not pass away, and his kingdom will not be destroyed."-Daniel 7:13, 14.
In this vision the "son of man," the designation given to this spirit creature in heaven, gains access to the very throne of Jehovah God. That means this spirit would become human, he would be born on earth as a "son of man," and thus rightly carry this title as well as "son of God." And the Ancient of Days, that is Jehovah God, would give him rulership, that the peoples, nations, and language groups should all serve even him.
So the Messiah will rule the entire earth. It is a heavenly kingdom. And Daniel also tells us that by means of the government in heaven all present human rulerships will be brought to nothing:
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In the days of those kings the God of heaven will set up a kingdom that will never be destroyed. And this kingdom will not be passed on to any other people. It will crush and put an end to all these kingdoms, and it alone will stand forever."-Daniel 2:44.
This is a very small, but very important glimpse of what the Hebrew Scriptures tell us about the Messiah and what he will do for humankind. There are many other prophecies!