Matthew 16:19 tells us: 
And I will give you the keys of the kingdom of heaven, and whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.

This verse is widely misinterpreted today.  The surface reader will automatically assume it speaks of binding the works of spiritual forces.  However, as with most of God's word a deeper study is required to derive the intended meaning from the verse. 

16:19 the keys of the kingdom of heaven
These represent authority, and Christ gives Peter (and by extension all other believers) authority to declare what was bound or loosed in heaven.  This echoed the promise of John 20:23.

John 20:23 
If you forgive the sins of any, they are forgiven them;  if you retain the sins of any, they are retained. 

This verse does not give authority to Christians to forgive sins.  Jesus was saying that the believer can boldly declare the certainty of a sinner's forgiveness by the Father because of the work of His Son if that sinner has repented and believed the gospel.  The believer with certainty can also tell those who do not respond to the message of God's forgiveness through faith in Christ that their sins, as a result, are not forgiven.

Christ gave the disciples authority to forgive or retain the sins of people.  All this must be understood in the context of 18:15-17, where Christ laid out specific instructions for dealing with sin in the church. 
The sum of it all means that any duly constituted body of believers, acting in accord with God's Word, has the authority to declare if someone is forgiven or unforgiven.   The church's authority is not to determine these things,  but to declare the judgment of heaven based on the principles of the Word.  When they make such judgments on the basis of God's Word, they can be sure heaven is in
accord.  In other words,  whatever they "bind" or "loose" on earth is already "bound" or "loosed" in heaven.  When the church says the unrepentant person is bound in sin, the church is saying what God says about that person, or the actual condition of that person.  When the church acknowledges that a repentant person has been loosed from that sin, God agrees. 

Aside...

Matthew 16:19 cannot mean, as the Pentecostal/Charismatic movement would have you to believe, that we have the power to bind the works of satanic forces, or that we even should.

We must also take into account other parts of scripture where we are taught and admonished not to have anything to do with demonic forces, not even railing against them, i.e. arguing with them, or accusing them, or undermining their intentions. 

As if the actual explanation above about binding and loosing were not enough, we have an abundance of other proofs that if "binding and loosing" meant that we could bind satanic forces, then the question would have to be asked, why then do the scriptures contradict themselves. 

Because elsewhere in scripture we find... 

2Peter 2:10  But chiefly them that walk after the flesh in the lust of uncleanness, and despise government. Presumptuous are they, selfwilled, they are not afraid to speak evil of dignities. 
 11 Whereas angels, which are greater in power and might, bring not railing accusation against them before the Lord.

Basically, the verse above tells us that powerful Angels do not even accuse or rail against  spiritual forces.  And it goes on to say before the Lord.  This is because the Lord God is in charge, and how can we humans know if God may be using satanic forces at His will to do His will in various situations? Therefore, if God is using satan or a demon to carry out something for His purpose, and we rail against that demon, do we not rail against an act of God? 

Also... 

 2Cor 12:7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure. 
 8 For this thing I besought the Lord thrice, that it might depart from me. 
 9 And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. 

In 2 Corinthians 12:7, the Holy and Saved Apostle Paul teaches us that a messenger of Satan came to buffet him, (or to harass Paul, or to harm him in some way).  Notice what Paul did about it.  He went to the Lord 3 times, asking the Lord to make it depart from him.  Paul realized that this messenger of satan may have been serving a specific purpose because Paul comments: "lest I should be exalted above measure" Paul did not dare to rail against this demon, or to be accusatory of it, or even to tell it to depart from him, but he went to God about it.  Always remember, it is God who is in charge, He is our boss, we 
do not take matters into our own hands, but we go to God about them.