Passage
To him therefore who knows to do good, and doesn’t do it, to him it is sin.
To him therefore who knows to do good, and doesn’t do it, to him it is sin.
James 4:15 For you ought to say, “If the Lord wills, we will both live, and do this or that.”
James 4:16 But now you glory in your boasting. All such boasting is evil.
James 4:17 To him therefore who knows to do good, and doesn’t do it, to him it is sin.
The verse centers on "therefore", "knows", "good", and "doesn". It is saying that the verse draws attention to "therefore" and "knows", so its meaning should be read from those terms before moving to application.
The prior verse says "But now you glory in your boasting...", giving immediate footing for "therefore" and "knows". In James context, the local focus is Christ, faith, and discipleship.
A plain takeaway is to answer the verse's own emphasis on "therefore" and "knows" with trust shaped by these words, not by a vague optimism outside the passage.