Verse 17: Therefore (Luke 12:47; John 9:41; 15:22; Rom. 1:20, 21, 32; 2:17, 18, 23) to him that knoweth to do good, and doeth it not, to him it is sin.
[Therefore, to the one knowing to do good (that is, a good to be done by him: For he speaks of a good practical and owed [Estius]), and not doing, it is sin to him] The αὐτῷ, to him, is redundant here, because εἰδότι, to the one knowing, preceded (Piscator). It is spoken comparatively, as in John 9:41. The sense is the same as in Luke 12:47. See what things were said by us in both places, and set those things over against those that think that a man always and necessarily wills according to the rule of that which is in his intellect: which I think to be refuted sufficiently by everyone’s internal experience. The Syriac reads εἰδότι οὖν τὸ καλὸν, to the one knowing the good, without ποιεῖν, to do, not incorrectly. Thus Lacon concerning the Athenians, They know what would be right, but they are not willing to do it (Grotius). Seneca, Epistle 95, After the learned have come forth, the good are wanting (Gataker). He uses sin emphatically, as in John 15:22 (Estius), whether κατ᾽ ἐξοχὴν, par excellence, or by Synecdoche, for scandal (Gomar), or for sin more grievous or greater: in other respects, also the ignorant sin, etc. (Estius). Whoever, having been admonished, sins, his sin is רָמָה/high, ὑπερηφανία/arrogance, Numbers 15:30 (Grotius). Now, this sentence is referred, either 1. to what immediately precedes. They were able to say, We know that without the nod of God we can do nothing, and this we presuppose in all things. But, says he, it is not enough to know this, unless ye apply it in deed, and thus speak habitually at the opportune time (Estius). James intends this: Ye have been admonished by me: ye are not able to plead ignorance. If ye should say any such thing after this, the fault shall be the greater (Grotius). Or, 2. which is more probable, to all the admonitions of this and the preceding chapter, which he concludes with this epiphonema; that is to say, I have now sufficiently admonished you concerning all these things (Estius); they are well known to you (Menochius). So then, if ye fail to obey (Estius), and to take pains (Menochius), it shall be a matter of great guilt to you, since ignorance will not excuse you (Menochius, similarly Estius).
Therefore, etc.: 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, John 9:41; John 15:22, 24.
 Greek: εἰδότι οὖν καλὸν ποιεῖν καὶ μὴ ποιοῦντι, ἁμαρτία αὐτῷ ἐστίν.
 James 4:17: “Therefore to him that knoweth (εἰδότι) to do good, and doeth it not, to him (αὐτῷ) it is sin.”
 Numbers 15:30: “But the soul that doeth ought presumptuously (בְּיָ֣ד רָמָ֗ה, with a high hand; ἐν χειρὶ ὑπερηφανίας, with a hand of arrogance, in the Septuagint), whether he be born in the land, or a stranger, the same reproacheth the Lord; and that soul shall be cut off from among his people.”
 That is, a succinct summary.