James 5:14–18:

14 Is anyone among you sick? Let him call for the elders of the church, and let them pray over him, anointing him with oil in the name of the Lord. 15 And the prayer of faith will save the one who is sick, and the Lord will raise him up. And if he has committed sins, he will be forgiven. 16 Therefore, confess your sins to one another and pray for one another, that you may be healed. The prayer of a righteous person has great power as it is working. 17 Elijah was a man with a nature like ours, and he prayed fervently that it might not rain, and for three years and six months it did not rain on the earth. 18 Then he prayed again, and heaven gave rain, and the earth bore its fruit.

Sinclair Ferguson on “the prayer of faith”:

. . . in the past century and a half, much has been written and said particularly about “the prayer of faith.” The focus has been on mountain-moving prayer by which we simply “claim” things from God with confidence that we will receive them because we believe that He will give them.
   But what exactly is the prayer of faith?

Association with the Dramatic

Interestingly, it is in the letter of James (who has so much to say about works) that the term occurs. It climaxes the marvelous teaching on prayer that punctuates the entire letter (see 1:5–8; 4:2–3; 5:13–18). . . .
   Elijah’s praying is used as an example not because it produce miracle-like effects but because it gives us one of the clearest of all illustrations of what it means for anyone to pray with faith: it is believing God’s revealed Word, taking hold of His covenant commitment to it, and asking Him to keep it.

The Prayer of a Righteous Person

Shutting up the heavens was not, after all, a novel idea that originated in the fertile mind of Elijah. In fact, it was the fulfillment of the promised curse of the covenant Lord: “If you do not obey the Lord your God . . . these curses will come upon you. . . . The sky over your head will be bronze, the ground beneath you iron. The Lord will turn the rain of your country into dust and powder” (Deut. 28:15, 22–24, NIV). . . .
   This, then, is the prayer of faith: to ask God to accomplish what He has promised in His Word. That promise is the only ground for our confidence in asking. Such confidence in not “worked up” from within our emotional life; rather, it is given and supported by what God has said in Scripture.
   Truly “Righteous” men and women of faith know the value of their heavenly Father’s promises. They go to Him, as children do to a loving human father. They know that if they can say to an earthly father, “But, father, you promised . . . ,” they can both persist in asking and be confident that he will keep his word. How much more our heavenly Father, who has given His Son for our salvation! We have no other grounds of confidence that He hears our prayers, we need none.

Legitimate Prayer

. . . Some Christians find this disappointing. It seems to remove the mystique from the prayer of faith. Are we not tying down our faith to ask only for what God already had promised? But such disappointment reveals a spiritual malaise: would we rather devise our own spirituality (preferably spectacular) than God’s (frequently modest)?
   The struggles we sometimes experience in prayer, then, are often part of the process by which God gradually brings us to ask for only what He has promised to give, the struggle is not our wrestling to bring him to give us what we desire, but our wrestling with His Word until we are illuminated and subdued by it, saying, “Not my will, but Your will be done.” Then, as Calvin again says, we learn “not to ask for more that God allows.”
   This is why true prayer can never be divorced from real holiness. The prayer of faith can be made only be the “righteous” man whose life is being more and more aligned with the covenant grace and purposes of God. In the realm of prayer, too (since it is a microcosm of the whole of the Christian life), faith (prayer to the covenant Lord) without works (obedience to the covenant Lord) is dead.

—Sinclair Ferguson, In Christ Alone: Living the Gospel Centered Life (Reformation Trust, 2007), 145–147.