I suspect that Nathaniel was rather young at the time that this event occurred: he shows the sort of cynicism which I associate with young men in their late teens at the same time as being rather pure or "free from guile," again something that I would associate with a young man trying to be innocent and pure of heart.  I do not think that Jesus is being sarcastic in what he says; this is just part of the amazing exchange of names and titles at the heart of this reading and the one preceding it.

What happened under the fig tree is never going to be completely clear to us.  From what Jesus says and from Nathaniel's reaction we can surmise that it was a very private and personal matter, one in which Nathaniel struggled and won in what was primarily a matter of honesty or straightforwardness, maybe of interior fidelity to God's love.  Jesus's next words are a matter of encouragement, a calling the young man to great deeds.  Jesus will later go on to expand the "angels ascending and descending" to say that "no one has gone up to heaven except the one who has come down from heaven" (John 3:13) and to say "I am the way" (that leads to the Father) (John 14:6).

The question is, as always, what this means for us.  I think that we too are called to struggle with our devils, especially internally, so as to be pure of heart before God even more than before other people.  We need to review our values, our excuses, the things that we wish to hide from, and the things that we repeat to ourselves about our judgements of other people, their motives, why we have trouble with them, etc.  This is the fig tree where we wrestle with ourselves to become truly gentle and loving, without guile.

Our struggle opens the door to the Lord, letting him reveal himself to us as our Way to God.  His coming to us will still be unexpected: we know not the day nor the hour, not the situation nor the messenger --- and it just might be one of those people from Nazareth that we have been spurning...