"Jesus: 'I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven' (Matthew 16:19). The imagery is straight forward: entrance into the kingdom of heaven - coming under the reign and rule of God in this life and in the life to come - is through the narrow door (Luke 13:24). The kingdom can be opened and the kingdom can be closed. With so much, everything in fact, riding on our entrance into the kingdom, we would do well to know how the door is opened and closed. The catechism mentions two keys of the kingdom: preaching and discipline." Kevin DeYoung. The Good News We Almost Forgot. 151.

Question 83: What are the keys of the kingdom?

Answer: The preaching of the holy gospel and Christian discipline toward repentance. Both preaching and discipline open the kingdom of heaven to believers and close it to unbelievers. 

Question 84: How does preaching the gospel open and close the kingdom of heaven?

Answer: According to the command of Christ: the kingdom of heaven is opened by proclaiming and publicly declaring to all believers, each and every one, that, as often as they accept the gospel promise in true faith, God, because of what Christ has done, truly forgives all their sins. The kingdom of heaven is closed, however, by proclaiming and publicly declaring to unbelievers and hypocrites that, as long as they do not repent, the anger of God and eternal condemnation rest on them. God's judgment, both in this life and in the life to come, is based on this gospel testimony. 

Question 85: How is the kingdom of heaven closed and opened by Christian discipline?

Answer: According to the command of Christ: those who, though called Christians, profess unchristian teachings or live unchristian lives, and after repeated and loving counsel refuse to abandon their errors and wickedness, and after being admonished by the church and it's officers, fail to respond to their admonition - such persons the officers exclude from the Christian fellowship by withholding the sacraments from them, and God Himself excludes them from the kingdom of Christ. Such persons, when promising and demonstrating genuine repentance are received again as members of Christ and of His church.