Passage
To him therefore who knoweth to do good and doth it not, to him it is sin.
To him therefore who knoweth to do good and doth it not, to him it is sin.
James 4:15 For what is your life? It is a vapour which appeareth for a little while and afterwards shall vanish away. For that you should say: If the Lord will, and, If we shall live, we will do this or that.
James 4:16 But now you rejoice in your arrogancies. All such rejoicing is wicked.
James 4:17 To him therefore who knoweth to do good and doth it not, to him it is sin.
The verse centers on "therefore", "knoweth", "good", and "doth". 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 you rejoice in your arrogancies...", 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.