“And since when does the OT lead one to believe in Jesus Christ without the New Testament?”
Act 17:2-3, “And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures,opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ.”
This is the message that Paul preached in the synagogues, including Berea. He preached that Jesus was the Messiah (Christ)and they would have searched the Isaiah and Zechariah prophecies to see if it was as Paul interpreted the scriptures concerning a “suffering Messiah”.
YES! And thank you for the scriptures! Maranatha
Maybe some have believed Paul's hyperbolic interpretation of Isaiah, but Judaism soundly rejected and rejects that the "Suffering Servant" is a man, much less a man called Jesus, let alone God, precisely based on the OT scripture, just as it rejected the Christian interpretation of Ps 110:1 a fabricated corruption.
The problem, of course, is that we have no one else to tesitify of what transpried from the opposing camp, and that most Christians simply take Paul on his word as a matter of "fact."