Expositor's Bible Commentary Commentary James 4:17

"To him therefore that knoweth to do good, and doeth it not, to him it is sin." — James 4:17 (ASV)

Although this statement may apply to any number of situations, James intends it to refer to the immediately preceding context. Perhaps this is a maxim that means something like “You have been fully warned,” as if James were saying, “Now that I have pointed the matter out to you, you have no excuse.” Knowing what should be done obligates a person to do it.