Where does the diatribe with the imaginary interlocutor that begins in v. 18 end?
An imaginary “someone” (τις) addresses James in v. 18. In doing this James has introduced a dialog with a straw man (an interlocutor) as his chosen form of diatribe. We can assume that at some point James responds to this interlocutor with a rebuttal otherwise it would be a failed attempt to “demolish an opposing argument” (Dowd, “Faith” in Expositor, 198). We can also assume that at some point James ends this “back-and-forth” with this interlocutor since v. 26 is clearly a summary and Jas. 3:1 represents a significant shift in thought.
The first interpretive method takes vv. 18-25 as a unit that represents one argument between James and the interlocutor. This is Burdick’s chosen method of interpretation as he sees Rahab’s example of faith as complementary to Abraham’s and therefore subsumed into the argument with the interlocutor (Burdick, James, 185). Blue also seems to endorse this interpretation though it may be that he finds the end of the diatribe exegetically insignificant and instead chooses to focus on the examples (Blue, James, 826).
While taking the two examples as a single unit seems to be the logical decision there is a significant clue in the Greek text that helps determine the end of the diatribe. Whereas, beginning in v. 18, the verbs of direct address are all singular, James shifts his attention to the audience with “You see …” (ὁρᾶτε). Many commentators are quick to pick up on this transition and point to the shift in attention (Moo, James, 114; Adamson, James, 132; Davids, James, 78).
Thus it seems that James’ intent was to address an anticipated objection with this diatribe and used Abraham’s example to rebuff the objection and derive the theological principle found in vv. 22-23. But why would the example of Abraham’s faith adequately rebuff the objection of the interlocutor but not that of the very real audience? Perhaps James anticipated objections that Abraham is a difficult example to follow and used the example of Rahab to give the congregation no excuse to not act on faith.
(taken from my answer here)