The OT saints did believe in Christ to the fullest revelation of Him that they had ... The Jews still believe in Christ, that is they believe in an anointed Messiah.
A temporary blindness has come upon them according to Paul.
They are yet to be truly converted. When they are they shall reign with Christ on earth during a 1, 000 years.
Only the Elect will reign with Christ and they are not just Jew's ..The Jew's make up only 2 houses .
Now let's look at this blindness
Romans 11:25 "For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fullness of the Gentiles be come in."
The blindness is the stupor that God sent upon those that cannot understand the plan of God, and see the prophetic signs around them. Remember Luke 21:24; "And they shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled." Christ is going to return to earth at the fullness of the Gentiles.
What happens then?
Romans 11:26 "And so all Israel shall be saved: as it is written, "There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:"
Romans 11:27 "For this is My covenant unto them, when I shall take away their sins."
This happens at the end of the Millennium age, not now, nor in the flesh. Why will this be? Because in the Millennium age all souls will be in their incorruptible bodies, and they will know Satan for what he is, as liar and deceiver, and they will have known Jesus Christ personally for that thousand year period.
This is the same as Paul wrote in Thessalonians
II Thessalonians 2:10 "And with all deceivableness [deceit] of unrighteousness in [for] them that perish; because they received not the love of the truth,"
II Thessalonians 2:11 "And for this cause God shall send them strong delusion [send them a working of error], that they should a lie;"
Jew's will NOT be saved just because they are Jew's ALL must believe in Christ , one can't wait till Christ returns and then say I believe .
They were broken off of their own tree ,whom is the Tree of Life and the only way to get grafted back on is to believe in Christ, Period!!!