These verses come immediately after Jesus' teaching about the role of the church in leading errant believers to repentance – firstly by private conversation, then by the added testimony of others and, if there is no repentance, the problem should be exposed to the church and the sinner excluded from fellowship.
 
The Lord Jesus added to that teaching that the gathered church has a divinely appointed contribution to make. When believers agree together with the Lord and pray, then He takes action. He restrains the tide of evil which comes from those who refuse to repent, and releases a tide of blessing and restoration to those who do repent.
 
God's people, their agreement and their prayers do make a real difference, as God directs. As the Holy Spirit moves their hearts, they find themselves working with God to achieve His purpose of repentance and restoration. As they pray, He releases repentant believers from their captivity to sin (Acts 8:21-24). On the other hand, as the church agrees together and prays, persistent sinners are deprived of the security of the fellowship and can no longer expect to be comforted in the church, until they choose to repent (1 Corinthians 5:3-5).
 
The attitude, so common in Western churches, that the church has no right to interfere with the way a dissident believer wants to go, is not supported by these verses. Indeed if the church is not concerned that the Name of the Lord is being slandered by the ungodly behaviour of a believer, the church is spiritually sick. But these verses are not an instruction to go around proudly criticising other believers. Jesus wanted the churches to know that they must not stand by when a person professes to believe the gospel but refuses to repent of ungodliness. The Lord wants churches to accept a measure of responsibility for the spiritual health of their congregation and to pray about those matters which diminish the gospel and degrade the sinner. When God's people pray, God works powerfully in His people.