Until now, according to John's Gospel, Jesus has been with John the Baptist.  He has already called his first two disciples: one was one of John's disciples, Andrew, who then brought his brother Simon to meet Jesus. Now Jesus moves away from John the Baptist and sets off towards his own ministry in Galilee, where he finds Philip and calls him to follow.

v.44
Now Philip was from Bethsaida, the city of Andrew and Peter.

If Philip came from Andrew and Peter's home town, it is likely that Jesus may have discussed possible disciples with them on their way back to Galilee.  Perhaps they introduced Philip to Jesus.  So, in this way the theme of Jesus' followers witnessing to their friends is continued.  Here is an example for all followers of Christ.

v.45
Philip found Nathanael and said to him, "We have found him about whom Moses in the law and also the prophets wrote, Jesus son of Joseph from Nazareth." 
NEB.  Philip went to find Nathanael, and told him, 'We have met the man spoken of by Moses in the Law, and by the prophets: it is Jesus son of Joseph, from Nazareth.

The theme continues.  Philip deliberately goes to look for Nathanael to tell him about Jesus.  When we have a relationship with the living Christ, it is not something to keep to ourselves.  Philip may have been enthusiastic about his new friend, but he did not go out into the streets and accost everyone he came across to try to drag them to meet Jesus.  He took the news to someone he knew would be open to hearing the message.  In the same way, we need to be sensitive in the people we approach and the way we invite them into a relationship with Christ.  It is important to be enthusiastic about the good news we bring, but we also need to be wise and allow the Holy Spirit to direct us in our witness.

Philip seems to recognise that Jesus is the person who has been prophesied about in the Jewish scriptures, although he doesn't actually call him the 'Messiah'

v.46
Nathanael said to him, "Can anything good come out of Nazareth?" Philip said to him, "Come and see." 

Nathanael is a 'straight-down-the-line' person.  He knows the reputation of Nazareth.  It is understandable that he might doubt that anyone from Nazareth - a country 'hick' town - could be sent by God to do anything important, let alone to be the Messiah. Philip sensibly does not try to argue.  He simply says, "Come and see for yourself"   He has met Jesus and he knows what kind of impact meeting Jesus has.

Quest Study Bible note, (p 1467):  Nathanael had good reasons to question the credentials of someone from Nazareth touted as the Messish.  It was an obscure town never mentioned in the Old Testament.  It also had become a barracks for Roman troops who brought with them their gods, their sins and their oppression of the Jews.  These factors made it seem improbable that it could be the hometown of the Messiah.

v.47
When Jesus saw Nathanael coming toward him, he said of him, "Here is truly an Israelite in whom there is no deceit!" 

Jesus recognised Nathanael for what he was - a totally honest man.  God approves of our being totally honest with him, even if we are angry or disapprove of something he does.  It is only when we are thoroughly open and honest with God that we are able to be taught by him and used for his service.  We can never fool God, but in trying to fool him, we are likely to mislead ourselves.  Dishonesty before God means that we are focussed onn ourselves and our own interests and not on him.  It also means a lack of trust in him to be able to accept us as we are.

v.48
Nathanael asked him, "Where did you get to know me?" Jesus answered, "I saw you under the fig tree before Philip called you." 

Nathanael was taken aback by Jesus' comment.  Perhaps he had a reputation for being brutally honest and knew it.  I wonder if Jesus' comment may have been said in a slightly teasing manner - as he often seemed to do with his disciples - yet said in a way that showed approval of Nathanael's total honesty.  How do you know about me? would be a natural question to ask someone who seems to know you by reputation. (How often has someone said, "I've heard about you" and our response is "I hope you heard good things.")  

Jesus' answer to Nathanael seems a bit strange to us, but it obviously made sense to Nathanael. We don't know which fig tree Jesus is talking about, what Nathanael was doing there, in what circumstances Jesus saw him, or how whatever incident it was showed Jesus that Nathanael was so honest.

Wherever we are, whatever we do or say, someone is watching us and making judgements about what kind of people we are.  If we are to be witnesses to the good news of Christ, we must be careful that our actions and words show our integrity.  It is not only God who sees our actions.  God knows our intent: others don't.

v.49
Nathanael replied, "Rabbi, you are the Son of God! You are the King of Israel!" 

Nathanael goes further than Philip and acclaims Jesus as  Son of God and King of Israel (or Messiah).  This might seem impulsive and premature to us, coming so early in the gospel story.  The other gospels do not record people as recognising Jesus for who he really was so early in his ministry.  Nathanael's recognition here comes long before Peter recognises Jesus as Messiah (Mark 8:29; Matthew 16:16; Luke 9:20).  Just as Peter's recognition was inspired by the Holy Spirit, so must Nathanael's have been.  I can imagine Nathanael, as he blurted out that statement, asking himself, 'Where did that come from?'

v.50
Jesus answered, "Do you believe because I told you that I saw you under the fig tree? You will see greater things than these." 
NIV:  Jesus said, "You believe because I told you I saw you under the fig tree.  You shall see greater things than that."

Everyone is different.  We come to believe in Christ in different circumstances and for different reasons.  Nathanael believed because of Jesus' insight into his character.  Thomas, after the resurrection, believed only when he could touch the risen Christ (John 20: 24-29).   How we come to believe is not important.  The crucial issue is that we believe.  Jesus promised Nathanael that he and the other disciples would see more wonderful things than simply special insight into their own personalities - and they did!

v.51
And he said to him, "Very truly, I tell you, you will see heaven opened and the angels of God ascending and descending upon the Son of Man." 

Nathanael must have wondered what Jesus meant by this prediction.  We know that the disciples did see a time when people everywhere would have opportunity to enter into a close personal relationship with God himself because of the death and resurrection of Jesus.  The picture painted by this statement reminds us of Jacob's dream of heaven opening and angels going up and down a 'ladder' or ramp between earth and heaven (Genesis 28: 12) and the Lord personally bringing a special message to Jacob.

New Bible Commentary,( p 1029):  The greater things of v 50 are explained by v 51, which speaks of the development of spiritual vision. ... The meaning of the statement is that heaven is now opened for continuous communication with people, the representative of whom is Christ himself under the title Son of Man.  It is remarkable that this title is substituted for Nathanael's Son of God, for this shows that the human aspect of Jesus is as important as the divine.