43 The following day Jesus wanted to go to Galilee, and He found Philip and said to him, “Follow Me.” 44 Now Philip was from Bethsaida, the city of Andrew and Peter. 45 Philip found Nathanael and said to him, “We have found Him of whom Moses in the law, and also the prophets, wrote—Jesus of Nazareth, the son of Joseph.”
46 And Nathanael said to him, “Can anything good come out of Nazareth?”
Philip said to him, “Come and see.”
47 Jesus saw Nathanael coming toward Him, and said of him, “Behold, an Israelite indeed, in whom is no deceit!”
48 Nathanael said to Him, “How do You know me?”
Jesus answered and said to him, “Before Philip called you, when you were under the fig tree, I saw you.”
49 Nathanael answered and said to Him, “Rabbi, You are the Son of God! You are the King of Israel!”
50 Jesus answered and said to him, “Because I said to you, ‘I saw you under the fig tree,’ do you believe? You will see greater things than these.” 51 And He said to him, “Most assuredly, I say to you, hereafter you shall see heaven open, and the angels of God ascending and descending upon the Son of Man.”
It's necessary to remember that at this time, Jews were earnestly expecting the arrival of the Messiah. The Aramaic term Messiah and the Greek word Christ both mean the same thing - the Anointed One, referring to a person who has been anointed to fulfil a certain office or function. The Messiah was prophesied in the Jewish Scriptures (the Old Testament), and while many Jews believed He would free them from the Romans and set up the kingdom of God on earth, Jesus' message was that He had actually come to make a sacrifice for the sins of humanity.
In the passage above, it's interesting that Jesus called Philip directly, and then Philip found Nathanael and told him about the Messiah. A similar thing happened in verse 41, where Andrew relays the news to his brother Simon (Peter). Here is a pattern of evangelism which all believers should emulate. As soon as they began following Jesus, both Andrew and Philip went to tell their family and friends (and later everybody) about Him. If we have believed in Him, then we should be doing the same. God has commanded us to do so (Matthew 28:19-20), but this is not a work we have to fulfil in order to gain His approval - if it were, then I would be in big trouble, as there have been countless times when I've been too cowardly or too comfortable to preach the Gospel. Instead of a "good work", our obedience to the Lord's instructions to evangelise should be motivated by a clear understanding and appreciation of the Gospel itself. If we have studied the doctrine of salvation by grace through faith alone, then we will understand that we can and should do nothing to add to Christ's work on the cross, and we will then serve Him based on our love and gratitude. It will also become clear that we were once under God's judgement just as all unbelievers are, and that it is only by His grace that we have been saved, not because we are better than anyone else. Finally, we should have a clear understanding of what we have been saved from - eternal torment in hell - and the realisation that this will be the destination of those who do not repent and trust in Jesus. All of this motivates us to make the good news of Jesus Christ - the Messiah - known to others.
Another interesting part of this passage is Nathanael's conversation with Jesus. On being told that the Messiah has come, Nathanael is at first somewhat sceptical. However, he is willing to put Jesus to the test, and it may be because of this that the Lord commends him for his honesty in verse 47. Unlike the Pharisees, Nathanael did not allow his own interests or biases to prevent him from discovering and believing the truth. Jesus then uses His supernatural knowledge to prove that He really is the Messiah, by revealing that He had some knowledge of Nathanael that nobody else had. This prompts Nathanael to affirm Jesus as the Son of God, and therefore as his personal Saviour. However, notice that if Nathanael had not been willing to consider the Lord's claims, no amount of miracles could have convinced him. The Pharisees saw a lot of amazing wonders and yet they still rejected Christ.
I couldn't leave this passage without commenting on Jesus' words in verse 51. By saying that Nathanael and the other disciples would see heaven opened, He was pointing out that they would have access to divine revelation through His Word. The image that Christ used here was an allusion to Jacob's dream recorded in Genesis:
12 Then he dreamed, and behold, a ladder was set up on the earth, and its top reached to heaven; and there the angels of God were ascending and descending on it. (Genesis 28:12)
In Jacob's dream, God showed that He was involved in the affairs of humans, and had a particular plan for Jacob and his descendants. However, in the picture offered by Jesus, the Son of Man becomes the ladder. This clearly demonstrates that Jesus is the means of access between God and humanity. He performs this function through the cross, on which He died to pay the penalty for our sin, and thereby made peace with God on our behalf.