After exhorting his readers on the importance of humility (James 4:13–16), James warns, “To him who knows to do good and does not do it, to him it is sin” (James 4:17, NKJV).
Throughout his letter, James challenges his readers to action. They should be doers of the word and not just hearers (James 1:22). If people are hearers but not doers, then they are deceiving themselves. They should speak and act as people who are accountable (James 2:12). Because faith without works won’t deliver a person in need and is of no use in resolving that situation (James 2:14), one ought to take action and meet the needs (James 2:16). In fact, James goes so far as to assert that works are necessary for a person to be justified in the sight of other people (James 2:24); works are a sign of one’s salvation. In the same way, it is sin to him who knows to do good and does not do it (James 4:17).
James challenges his readers that, if they think themselves wise, they should show it by deeds of good behavior in the humility of wisdom (James 3:13). With point after point, James calls people to take action—to do what they know is right. Do they wish to be close to God? Then they should draw near to Him (James 4:8). Do they wish to be exalted? Then they should humble themselves (James 4:10). With each desired result, there is a necessary action, and we are accountable for doing what we ought to do. It is sin to him who knows to do good and does not do it.
In 1 Corinthians Paul deals with similar issues. He instructs the Corinthians believers to be sensitive to conscience issues. Paul writes that all things are lawful, but not all things are profitable (1 Corinthians 10:23). Because of this principle, people should be seeking the good of the other, even being willing to give up their own liberties for the sake of others. Instead of seeking their own good, they should seek the good of their neighbor (1 Corinthians 10:24). They had freedom to eat anything sold in the marketplace (even meat sacrificed to idols, see 1 Corinthians 10:26). But that freedom should be voluntarily curtailed if someone raised an issue for conscience sake (1 Corinthians 10:28). Like James said, it is sin to him who knows to do good and does not do it (James 4:17)—even on conscience issues.
Paul adds that it is good not to eat or drink anything that might cause a brother to stumble (Romans 14:22). He who doubts that he ought to eat, for example, is condemned by his eating because he eats in violation of his own faith. That which is not of faith is sin (Romans 14:23). This corresponds to what James is talking about. We are accountable for what we do, and, as we learn what we ought to do, we need to be doers and take action. We also need to be sensitive to those around us and seek their good. We need to be willing to limit our liberties where necessary in order to help those who may have issues of conscience.