What are the keys of the kingdom of heaven?
The preaching of the holy gospel, and Christian discipline, or excommunication out of the Christian church; by these two, the kingdom of heaven is opened to believers and shut against unbelievers.

How is the kingdom of heaven opened and shut by the preaching of the holy gospel?
Thus: when according to the command of Christ it is declared and publicly testified to all and every believer, that, whenever they receive the promise of the gospel by a true faith, all their sins are really forgiven them of God for the sake of Christ’s merits; and on the contrary, when it is declared and testified to all unbelievers, and such as do not sincerely repent, that they stand exposed to the wrath of God and eternal condemnation, so long as they are unconverted; according to which testimony of the gospel, God will judge them both in this and in the life to come.