Read: John 1:46-51
Nathanael answered him, "Rabbi, you are the Son of God! You are the King of Israel!" (John 1:49 ESV)
Jesus' prophetic display convinced Nathan to follow him. He exclaimed his astonishment by attributing three very Jewish titles to Jesus. He called him a rabbi, the Son of God, and King of the Jews.
Rabbi was a title. It was and still is a Jewish word for teacher. It was used frequently when speaking of Jesus. For good reason too. He was teaching the people of his day the truth of the kingdom of God in a way that baffled, angered, and shamed the religious elite, while encouraging and empowering the destitute and ignored. Jesus taught about a lot of things, but he demonstrated all of his instruction very pointedly by becoming the sacrifice to fulfill all of his earthly promises.
Son of God seems to be a really straight forward thing to call someone, and basically it is. It is a figure of speech that goes all the way back to the creation story when Adam is called the son of God. It is a theme that runs through the entire Old Testament, spinning out of the promise God made to Abraham that he would be an exalted father. Jesus is the ultimate fulfillment of those three words. He is the Son of God. He is God the Son. Essentially, all Old Testament themes speaking of sonship and family point ahead to Jesus. He is the fulfillment of that and so much more. And you and I can also be sons of God as we are adopted into the family of God through Jesus' sacrifice.
King of Israel was a curious thing to call Jesus. After all, the nation already had a king. This title declares two things. First, the man who sat on the throne at the time of Jesus was not the true king of Israel. He was not a descendant of David. No, Herod was a puppet king put in place by the Romans. Second, the title attributed to Jesus recognition of his lawful right to the throne, something he did not pursue at that time, as well as his role as an eternal king of God's people.
Ultimately Jesus is the supreme teacher, son, and ruler. In him we have revealed to us absolute truth. In him we find restoration into the original family. In him we gain citizenship to a sovereign eternal kingdom.