John Calvin on James 2:19

19 Thou believest that there is one God. From this one sentence it appears evident that the whole dispute is not about faith, but of the common knowledge of God, which can no more connect man with God, than the sight of the sun carry him up to heaven; but it is certain that by faith we come nigh to God. Besides, it would be ridiculous were any one to say, that the devils have faith; and James prefers them in this respect to hypocrites. The devil trembles, he says, at the mention of God’s name, because when he acknowledges his own judge he is filled with the fear of him. He then who despises an acknowledged God is much worse.

Thou doest well, is put down for the purpose of extenuating, as though he had said, “It is, forsooth! a great thing to sink down below the devils.” The design of alluding to the faith of devils seems to have been this, to shew that though a good man may believe and tremble, yet if he does not obey God and do good works, he has no true evidence of faith. Obedient faith is that which saves, and not merely that which makes us tremble. The connection with the preceding verse seems to be as follows, —

In the former verse the boaster of mere faith is challenged to prove that his faith is right and therefore saving; the challenger would prove by his works. Then, in this verse, a test is applied — the very first article of faith is mentioned: “Be it that you believe this, yet this faith will not save you: the devils have this faith, and instead of being saved they tremble.

James 2:20-26

20 But wilt thou know, O vain man, that faith without works is dead?

20 Vis autem scire, O homo inanis! quod fides absque operibus mortua sit?

21 Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?

21 Abraham pater noster, nonne ex operibus justificatus est, quum filium suum Isaac super altare?

22 Seest thou how faith wrought with his works, and by works was faith made perfect?

22 Vides quod fides cooperata fuerit ejus operibus, et ex operibus fides perfecta fuerit?

23 And the scripture was fulfilled which saith, Abraham believed God, and it was imputed unto him for righteousness: and he was called the Friend of God.

23 Atque implenta fuit scriptura, quae dicit, Credidit Abraham Deo, et imputatum illi fuit in justitiam, et Amicus Deo vacatus est?

24 Ye see then how that by works a man is justified, and not by faith only.

24 Videtis igitur quod ex operibus justificatur homo, et non ex fide solum.

25 Likewise also was not Rahab the harlot justified by works, when she had received the messengers, and had sent them out another way?

25 Similiter et Rahab meretrix, nonne ex operibus justificata est, quum excepit nuntios, et alia via ejecit?

26 For as the body without the spirit is dead, so faith without works is dead also.

26 Quemadmodum enim corpus sine anima mortuum est, ita et fides sine operibus mortua est.