Passage
Therefore, to him that knoweth how to doe well, and doeth it not, to him it is sinne.
Therefore, to him that knoweth how to doe well, and doeth it not, to him it is sinne.
James 4:15 For that ye ought to say, If the Lord will, and, if we liue, we will doe this or that.
James 4:16 But now ye reioyce in your boastings: all such reioycing is euill.
James 4:17 Therefore, to him that knoweth how to doe well, and doeth it not, to him it is sinne.
The verse centers on "therefore", "knoweth", "well", "doeth", and "sinne". It is saying that the verse draws attention to "therefore" and "knoweth", so its meaning should be read from those terms before moving to application.
The prior verse says "But now ye reioyce in your boastings...", giving immediate footing for "therefore" and "knoweth". 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 "knoweth" with trust shaped by these words, not by a vague optimism outside the passage.