Again, normally I post the text of the passage in the post, but this passage is a little long so I will just link to it, here.
While this won't be a long post, I think its worth while to flesh out the main point of verses 16-21 a bit more fully. As Protestants I think we have a tendency to assume the doctrine of justification by faith without closing examining what the relevant texts actually say. I think this text is a classic case in point. We read that 'whoever believes in him shall not perish but have eternal life' and think, ok, case closed. But, as I pointed out in my last post, if you finish reading the paragraph you will get a different picture. Faith is presented as being the outcome of a life or character that is holy. Those who are holy come to the light, they come to Jesus because they have nothing to fear. There is nothing that they are afraid of being exposed.
Commonly Protestants will claim that the good works God requires flow from faith. John is saying just the opposite. The faith that God requires flows from good works. That's rather shocking and jarring to our Protestant sensibilities, but perhaps they need a little jolting.
As a result I think we need to consider a few things. First, the situation of the author of the second strata of the Gospel of John is clearly one of conflict. In all likelihood there had been a split with the Jewish community in its past, and probably there is a bit of polemic involved. Those in the synagogue who cast them out did so because they disagreed with the Johannine community on who Jesus was. The reason they got it wrong is because of their own moral failure. Their depravity prevented them from coming to Jesus. They had something to hide. So we do have to consider that this verse isn't really a blanket statement about the nature of justification, but a shot across the bow of a group that caused the author and his community pain.
Second, we do need to consider that different biblical authors may not always have the same perspective. John and Paul don't have to agree. Certainly John and the standard Protestant exegesis of Paul don't have to.
Third, and related, perhaps we need to reconsider our understanding of justification, salvation, faith, and works. What is the relationship of works and faith with justification and salvation? It's obvious that both faith and works are strongly related in some way with salvation. How related are each with justification? That's a little harder to tell. At the end of the day, this passage clearly underscores that works do play a role in final salvation for the author of John. Even accepting it as a polemical passage, we still have to admit that much. But that shouldn't be a surprise to us as that seems to be the case for all strands of early Christianity. How exactly they relate is an interesting discussion, but one best left for another time. My primary concern is to underscore that we need to think hard about how works relate to final salvation. A scheme that marginalizes them runs in to trouble all over, even in a passage like this that we might expect to be a bedrock of a faith alone approach.
 Or of the final editor, if one doesn't adhere strictly to von Wahlde's scheme.
 It need not have been a recent split to have been painful, memorable, and defining for the group.