Reason #1: The Bible says it!
John 20:31 "But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name."
Hebrews 1:1-2 "
John 3:16-18 "
And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
Reason #2: Jesus is the fulfillment of Prophecy
Here are a few examples...
Micah 5:2 “But you, Bethlehem Ephrathah,
The fulfillment is spoken of in Matthew chapter 2.