Either this may relate to all that the apostle had been before speaking of; q.d. I have admonished you of your duty, and now ye know what ye are to do, and therefore if you do it not it will be your sin: or, it may refer to what he was immediately before discoursing of, and may be spoken to prevent an objection. They might say, he taught them no more than what they knew already; and that they acknowledged God's providence in all things. To this he replies, that if they knew their duty, they ought to practise it, and so actually submit themselves and their affairs to the conduct of that providence; and their not doing it, now that they knew it, would the rather be their sin. To him it is sin; i.e. sin indeed, or (as we say) sin with a witness; a greater sin, and which hath more of the nature of sin in it, or is more highly aggravated, by being against knowledge, and so is punishable with severer vengeance, than if done out of ignorance, Luke 12:47. See the like expression, 1 Thessalonians 9:41, 1 Thessalonians 15:22,24.
Related Commentaries of James 4:17
James 4:17
17 Therefore to him that knoweth to do good, and doeth it not, to him it is sin.