There is some variety in what Christians have taught concerning this verse, but most seem to focus on it represent one or more of the following
- An allusion to Jacob's dream of a ladder (Genesis 28:10-22). These typically present Christ as the fulfillment of that dream.
- A specific prediction of an event in Christ's life that Nathanael would witness, especially
- Christ's Baptism
- Christ's Transfiguration
- Christ's Ascension
- A prediction of the Second Coming
- A general claim to divinity
There are some who mention Nathanael and that Christ has confirmed his confession "Rabbi, You are the Son of God! You are the King of Israel!", but that they usually move quickly on to interpreting Christ's words. I haven't found any that explicitly mention Nathanael's ultimate fate. Those that mention Nathanael tend to focus more on the miracles and wonders that Nathanael will witness.
Here are some of the highlights.
John was Patriarch of Constantinople around AD 400. He is well respected by most Christian traditions, but is particularly beloved by the Orthodox. In his 21st Homily on the Gospel of John, he focuses on how the angels serve Christ throughout His sojourn of Earth and that this is proof of Christ's Divine Sonship.
Persuading him by these words to own Him Lord also of the Angels. For on Him as on the King's own Son, the royal ministers ascended and descended, once at the season of the Crucifixion, again at the time of the Resurrection and the Ascension, and before this also, when they came and ministered unto Him, when they proclaimed the glad tidings of His birth, and cried, 'Glory to God in the highest, and on earth peace', when they came to Mary, when they came to Joseph. ... For of His sayings some had been proved, such as, Before Philip called you, under the fig-tree I saw you; others had yet to come to pass, and had partly done so, namely, the descending and ascending of the Angels, at the Crucifixion, the Resurrection, and the Ascension; and this He renders credible by His words even before the event.
Augustine of Hippo
Augustine was Bishop of Hippo around AD 400. He is among the most influential theologians in Christian history. In his 7th Tractate on John, Augustine refers back to Jacob and asserts that the dream pointed to Christ:
Jacob saw a ladder in a dream; and on a ladder he saw angels ascending and descending: and he anointed the stone which he had placed at his head. You have heard that the Messiah is Christ; you have heard that Christ is the Anointed. For Jacob did not place the stone, the anointed stone, that he might come and adore it: otherwise that would have been idolatry, not a pointing out of Christ.
He goes on to interpret the angels on the ladder as the Christian preachers. Nathanael probably did not understand it this way, but it is worth mentioning.
But what saw he then on the ladder? Ascending and descending angels. So it is the Church, brethren: the angels of God are good preachers, preaching Christ; this is the meaning of, they ascend and descend upon the Son of man.
Cyril of Alexandria
Cyril was Patriarch of Alexandria in the 400s. He was central to the defense of the Two Natures of Christ against Nestorius. His exposition may come the closest to commenting on Nathanael's salvation, but he seems to focus more on the evidence that will confirm Nathanael's confession rather than his final state.
Thou shalt be firmer unto faith, saith He, when thou seest greater things than these. For he that believed one sign, how shall he not by means of many be altogether bettered, especially since they shall be more wonderful than those now wondered at? ... Common now to all is the word which seals the faith of Nathanael. But in saying that angels shall be seen speeding up and down upon the Son of Man, that is, ministering and serving His commands, for the salvation of such as shall believe, He says that then especially shall He be revealed as being by Nature Son of God. For it is not one another that the rational powers serve but surely God.
His commentary then moves on to a discussion of Christ's miracles and ties these back to the wonders that Nathanael will see.
Aquinas was a Catholic priest and monk in the 1200s. He is considered a Doctor of the Church by the Catholic Church. Aquinas uses the passage to illustrate Christ's power and His two natures:
Because Christ spoke the truth about the past, it was easier for Nathanael to believe what he foretells about the future, saying, you will see. For one who has revealed the truth about things hidden in the past, has an evident argument that what he is saying about the future is true. He says, the angels of God ascending and descending on the Son of Man, because, in his mortal flesh, he was a little less than the angels; and from this point of view, angels ascend and descend upon him. But insofar as he is the Son of God, he is above the angels, as was said.
He goes on to agree with Augustine that the statement refers back to Jacob and a prefiguring of Christ:
According to Augustine, Christ is here revealing his divinity in a beautiful way. For it is recorded that Jacob dreamed of a ladder, standing on the ground, with "the angels of God ascending and descending on it". Then Jacob arose and poured oil on a stone and said, "Truly, the Lord is in this place". Now that stone is Christ, whom the builders rejected; and the invisible oil of the Holy Spirit was poured on him. He is set up as a pillar, because he was to be the foundation of the Church: “No one can lay another foundation except that which has been laid” (1 Cor 3:11). The angels are ascending and descending inasmuch as they are ministering and serving before him. So he said, Amen, amen, I say to you, you will see the heavens opened, and so forth, as if to say: Because you are truly an Israelite, give heed to what Israel saw, so that you many believe that I am the one signified by the stone anointed by Jacob, for you also will see angels ascending and descending upon him
Calvin was one of the principle theologians of the Reformation. He takes the position that we should not spend too much time wondering about Nathanael but should instead learn what we can about the Kingdom of God:
They are greatly mistaken, in my opinion, who anxiously inquire into the place where, and the time when, Nathanael and others saw heaven opened; for he rather points out something perpetual which was always to exist in his kingdom. I acknowledge indeed, that the disciples sometimes saw angels, who are not seen in the present day; and I acknowledge also that the manifestation of the heavenly glory, when Christ ascended to heaven, was different from what we now behold. But if we duly consider what took place at that time, it is of perpetual duration; for the kingdom of God
To that end, Calvin understands the statement to refer to Jacob's dream being fulfilled in Christ, the only Mediator between heaven and earth.
They are said to ascend and descend, so as to be ministers of God's kindness towards us; and therefore this mode of expression points out the mutual intercourse which exists between God and men. Now we must acknowledge that this benefit was received through Christ, because without him the angels have rather a deadly enmity against us than a friendly care to help us. They are said to ascend and descend on the son of man, not because they minister to him, but because -- in reference to him, and for his honor -- they include the whole body of the Church in their kindly regard. Nor have I any doubt that he alludes to the ladder which was exhibited to the patriarch Jacob in a dream, for what was prefigured by that vision is actually fulfilled in Christ.
In conclusion, I don't think we can completely rule out the idea Christ was confirming Nathanael's salvation, but it seems that Christian teachers have not dwelt on this. Instead the focus, in one way or another, on Christ.