Old Testament prophecies that were fulfilled by Jesus in the New Testament helped his followers (and us) see that Jesus was (and is) who he said he was. Jesus said this to his disciples at the end of Luke’s gospel:
“'These are My words that I spoke to you while I was still with you that everything written about Me in the Law of Moses, the Prophets, and the Psalms must be fulfilled.' Then He opened their minds to understand the Scriptures.”
-Luke 24:44-45