See the story about Rich man & Lazarus. Why did Jesus teach that the Law and Prophets were sufficient for salvation?
Because they spoke of Him. Have you never read that "the testimony of Jesus is the Spirit of Prophecy"?
Why did he state that resurrection from the dead would not convince a person who rejected the OT? Is not the NT then icing on the cake?
The NT is hidden in the OT. Thus Paul tells us at 1 Cor 3 that the veil over the Jews which stops them from seeing Christ in the OT, is removed once they come to Christ.
Note then that he whose vail was removed every time he entered the inner court of the tabernacle to speak with God, is said to have preferred the reproach of Christ to the pleasures of the world.