A.T. Robertson 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)

To him that knoweth (ειδοτ). Dative case of second perfect participle ειδως (from οιδα), and with the infinitive to know how, "to one knowing how."

To do good (καλον ποιειν). "To do a good deed."

And doeth it not (κα μη ποιουντ). Dative again of the present active participle of ποιεω, "and to one not doing it." Cf. "not a doer" (1:23) and Mt 7:26.

Sin (αμαρτια). Unused knowledge of one's duty is sin, the sin of omission. Cf. Matthew 23:23.