John Gill 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)

Therefore to him that knows to do good
This may regard not only the last particular of referring all things to the will of God, the sovereign disposer of life, and all events, which some might have the knowledge of in theory, though they did not practise according to it; but all the good things the apostle had exhorted to, and the contrary to which he had warned from, in this epistle; and suggests, that a Gnostic, or one that knows the will of God, in the several branches of it, revealed in his word,

and does it not, to him it is sin :
it is a greater sin; it is an aggravated one; it is criminal in him that is ignorant of what is good, and does that which is evil, nor shall he escape punishment; but it is much more wicked in a man that knows what is right and good, and ought to be done, and does it not, but that which is evil, and his condemnation will be greater; see (Luke 12:47Luke 12:48) . The omission of a known duty, as well as the commission of a known sin, is criminal.