John Painter (The Quest for the Messiah, pp. 182-188) presents a persuasive argument for the later addition of John 1:51 to the unit that begins at 1:19 and ends with 2:11. The whole unit is about the quest for the messiah, and its fulfilment. The unit begins with the interrogation of John the Baptist by those sent from Jerusalem. They ask him, 'Are you the Christ?' (John is not "I AM"; compare 1:20, 21 with, for example, 8:28, 58) The next day (1:29; see also 1:43; 2:1 binding this whole unit together) John directs two of his disciples to Jesus, and they stay with him. (1:35-39) Andrew then brings his brother Cephas to Jesus, who renames him Peter. (1:40-42 compare Matt 16:18) Finally, we have Jesus' interaction with Nathaniel (1:43-51), where Jesus promises Nathaniel that he will see greater things than these. (1:50) It is possible that, originally, the narrative continued into John 2 (minus 1:51), with the story of the water into wine. (Jn 2:1-11) This miracle story culminates at 2:11, where the disciples of Jesus see this his first sign, and his glory, and they believed in him. At a later stage (so the persuasive argument goes) 1:51 was inserted, shifting the 'greater things than these' (1:51) from the miracle itself to the Son of Man's glorification and exaltation in the cross and resurrection. The signs in themselves are not the end point of faith. (See 6:30-34)
William Temple, in his Readings in St John's Gospel (First Series: Chapters I-XII), adds a different interpretation to Nathaniel's coming to faith. When Jesus says, "Do you believe because I told you that I saw you under the fig tree?" (1:50), the answer is 'No'. Jesus understands that Nathaniel believes because he wrestled with the origin of Jesus under the fig tree. (1:46). This is why Jesus says Nathaniel is a true Israelite without guile. Nathaniel has wrestled with God just as Jacob/Israel did before him.(Genesis 32:24-29) However, Jesus senses that Nathaniel does not have the deceptive character of Jacob. To emphasise this last point, Temple's translation of 1:47 is, "... truly an Israelite in whom there is no Jacob!" But, despite this twist in interpretation (again persuasive), Temple still thinks that Nathaniel's faith is, according to John, based on an inadequate foundation. It is a better one than faith in a miracle worker, but still not the faith of mature discipleship.