John Calvin Commentary James 5:15

"and the prayer of faith shall save him that is sick, and the Lord shall raise him up; and if he have committed sins, it shall be forgiven him." — James 5:15 (ASV)

But it must be observed that he connects a promise with the prayer, so that it is not made without faith. For he who doubts, as one who does not rightly call on God, is unworthy to obtain anything, as we have seen in the first chapter. Whoever then really seeks to be heard must be fully persuaded that he does not pray in vain.

As James brings before us this special gift, to which the external rite was only an addition, from this we learn that the oil could not have been rightly used without faith. But since it appears that the Papists have no certainty regarding their anointing, as it is clear that they do not have the gift, it is evident that their anointing is false.

And if he have committed sins. This is not added only for the sake of amplifying, as if he had said that God would give something more to the sick than health of body; but it is added because diseases were very often inflicted on account of sins. By speaking of their remission, he intimates that the cause of the evil would be removed. And we indeed see that David, when afflicted with disease and seeking relief, was entirely focused on seeking the pardon of his sins. Why did he do this? It was because, while he acknowledged the effect of his faults in his punishment, he considered that there was no other remedy than for the Lord to cease imputing his sins to him.

The prophets are full of this doctrine: that people are relieved from their evils when they are released from the guilt of their iniquities. Let us then know that the only proper remedy for our diseases and other calamities is to carefully examine ourselves, being eager to be reconciled to God and to obtain the pardon of our sins.