Suddenly James shifts his emphasis from whether we know God's will to whether we do God's will. Verse 17 seems at first not to fit the thrust of the paragraph. That, however, is a clue not that James is erratic in his thought but that we have not understood his meaning. The adverb oun ("then") provides grammatical evidence that James intends a connection in thought. He may have made a jump in his line of thought without articulating the intervening steps, but it is entirely consistent with the rest of the letter for James to tell his readers to carry out their inward attitude with outward actions. In fact, James capsulizes in this one verse much of what he has already taught in the letter. His double use of the verb poieo (to do and doesn't do) reminds his readers succinctly of his earlier emphasis on doing the word of God (1:22-25). The picture of one who knows the good he ought to do and doesn't do it recalls the earlier picture of one who finds the brother or sister in need but does not do the good that ought to be done (2:15-16). The label of "sin" (hamartia) is applied with all the severe warning about sin given earlier (1:15).
James fully expects that a humble attitude will be manifest in humble actions, and an arrogant attitude will be manifest in arrogant actions. It is natural for him now to be saying in 4:17: "Do not merely say that you want to know God's will or that you recognize your dependence on his will; look carefully at what God has already said about his will, and do that."
Failure to do what one knows to be God's will is the same arrogance that James has been describing in knowledge and attitude, now carried out in behavior. Indifference toward God's will is commonplace sin today, and Motyer comments on this verse that "the whole idea of sinning by default has never been given more pointed expression" (1985:163). As we have previously found in this letter, here James carries the issues of faith into the realm of active obedience.
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