Who is Jesus?

Jesus is the only begotten Son of God.  

How do we know this?

Because the bible tell us so.

Mathew 3:17

And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.

Mark 9:7

And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him.

John 3:16

For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

John 20:31

But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

How do we know what the bible say is true?

The Bible is written by Christians inspired by God.  God knew his Word was to be spread to the world.