Prayer……This places no limits on a believer’s prayers , as long as they are according to God’s will and purpose (see note on Mt 17: 20 ). This therefore means that man’s faith and prayer are not inconsistent with God’s sovereignty. And it is not the believer’s responsibility to figure out how that can be true, but simply to be faithful and obedient to the clear teaching on prayer, as Jesus gives it in this passage. God’s will is being unfolded through all of redemptive history, by means of the prayers of His people— as His saving purpose is coming to pass through the faith of those who hear the gospel and repent. Cf. Jas 5: 16 . Taken from comments in John MacArthur Study Bible.