Putting together an outline for the letter from James can be a challenge. And a quick survey of commentaries leaves us with a variety of suggestions.

Scot McKnight, quoting Duane Watson offers:

[James] is a Jewish-Christian work influenced by Hellenistic rhetoric, but is arranged overall in the topic-to-topic fashion of Jewish wisdom texts.

Even if the structure is hard to nail down, as we read through the letter of James we can see a theme emerge. James is writing to defend the idea that a genuine faith endures through trials and is demonstrated through good works. The good works that James emphasizes as evidence of genuine faith – social justice (1:27), our speech (1:26), and avoiding worldliness (1:27) – are summarized in the first chapter. Each of these topics receive more detailed treatment later in the work.

James also warns us not to be deceived. But what is it that we may be deceived about? In the larger context, it seems that James is warning us not to be deceived about who God is (1:16, 3:17) nor about our being a genuine disciple of Christ. True disciples are doers of the Word (1:22-25) , have a faith that is shown by good works (1:27; 2:17, 20), and in the meekness of wisdom (3:13) rather than worldliness (4:4).

Perhaps the verse that captures the theme best is a mashup of James 1:2-3 and 1:12

Consider it all joy, my brethren, when you encounter various trials, knowing that the testing of your faith produces endurance. … Blessed is a man who perseveres under trial; for once he has been approved, he will receive the crown of life

Trials are a process through which faith is tested. The testing can do one of two things. It can discover if faith that is claimed is genuine (2:26) or it can refine and strengthen the faith that is there resulting in our growth (1:4). The key word is the adjective δοκιμος, found in James 1:12 which is translated “approved” in the NASB.  The term (link) was often used to describe testing a coin to see if it was genuine or a counterfeit.

We can see the same idea in 1 Peter 1:6-7  where the testing of faith during trials is compared to gold. Here the noun (“proof”) and verb (“tested”) form of the word δοκιμος are used.

 In this you greatly rejoice, even though now for a little while, if necessary, you have been distressed by various trials, so that the proof of your faith, being more precious than gold which is perishable, even though tested by fire, may be found to result in praise and glory and honor at the revelation of Jesus Christ

Here is a rough outline of the letter based on this theme, or rather that highlights that this is a theme that James had in mind.

Other commentaries observe this theme in James as well.

In the MacArthur Bible Study Guide, there is an “emphasis on spiritual fruitfulness demonstrating true faith.”

If a person’s faith is genuine, it will prove itself during times of trouble, whatever the nature or source of the trouble may be.

J.A. Motyer, in The Message of James, identifies the themes of the letter as being centered around genuine faith, which is marked by growth and ethics (page 14-16).

Make sure your growth is a true, Christian development, and remember that it is by leaping life’s hurdles that you get to the tape.

…  James might ask, Did you in fact realize that the meeting of needs is not peripheral, nor optional, but central and obligatory to your faith?

While Scot McKnight sees numerous themes in the letter of James, the central theme for him is the broad topic of ethics. Within the section on ethics, McKnight does agree with the ideas presented by other commentators regarding ethics and good works as being evidence of a genuine faith (TNICNT page 46).

If one does not perform or live out the faith, one will not find eschatological salvation (cf. 2:14, 17, 18-19).  It is unwise to reify these terms and say one must have one or another, or even to say one must have all. Instead, each of these terms bring to expression a life that is lived properly before God if one is following the Messiah