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The people living under the OT were preached to for ages; they were told that a Savior would be born who would be their salvation. They longed for his arrival because their lives were so difficult. He was thought to be coming as a descendant of king David, who would settle it for once and for all, who would occupy the throne on which they wanted their earthly king to sit. He'd be a great king like they had with David. And his kingdom would never end; they were 'all in' for that.
He showed up and was a great disappointment; why else would they have done all they did to put him to death? They had no idea that they were fulfilling prophecy. His death would result in his resurrection, which would bring eternal life to all who believes in him. It had been a guarded secret, a mystery "kept in the dark for a long time, but now it's out in the open" (Col 1:26 Msg ).
The people he came to (his people, Israel) STLL (a lot of them) say that he was a fraud. They say they're still waiting for the one about which the prophet Isaiah and others preached.
The OT was a covenant between God and Israel. It had nothing to do with any other people. That is still today not understood by many people, in the church or outside of it. It's an important part of the history of all of mankind, both Jew and Gentile. You won't understand the new if you don't know the old. The OT is very important. If we neglect to study the OT, it's like trying to learn to read without first learning the alphabet.