Christianity is based on the idea that Jesus was God.
However Jesus did not think he was God
Therefore either christianity is wrong, or Jesus was.
Um Mathew, Mark, and Luke keep referring to Jesus as the Son of God.
So what? I'm my dad's son, that doesn't mean I'm my dad.
The story here is actually very interesting. See early Christians were actual very diverse in their interpretations of who Jesus was. From reconstructing the Gospels and letters of Paul in their original form along with the picture gained from other sources (mostly Christians writing stuff about how other Christians were heretics) Historians have learned that the first Christians like St Paul and the author of Mark were Jews who believed that Jesus was the messiah (as in a regular human God choose for an important task) who was God's
adopted son. Later in Matthew and Luke Jesus becomes not just the adopted son of God but his biological son like Heracles is to Zeus. It isn't until even later in John that Jesus is believed to be God himself.
Since the idea that Jesus was God was a later development in Christianity, we can assume that the original beliefs were closest the Jesus's own. So yeah Jesus would have thought of himself as the Jewish messiah, a new King David or Moses, but the idea that he was God he would have seen as blasphemous.
(For my sources, check out Dr Bart Ehrman's books, which this post is a highly oversimplified version of.)