Heidelberg Catechism · 1563

Q84. How is the kingdom of heaven opened and closed by the preaching of the gospel?

According to the command of Christ, the kingdom of heaven is opened when it is proclaimed and publicly testified to each and every believer that God has really forgiven all their sins for the sake of Christ's merits, as often as they by true faith accept the promise of the gospel. The kingdom of heaven is closed when it is proclaimed and testified to all unbelievers and hypocrites that the wrath of God and eternal condemnation rest on them as long as they do not repent. According to this testimony of the gospel, God will judge both in this life and in the life to come.
Answer · Heidelberg Catechism

Scripture proofs

  1. [1]
    Matthew 16:19

    And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven.

    John 3:31-36

    He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all. And what he hath seen and heard, that he testifieth; and no man receiveth his testimony. He that hath received his testimony hath set to his seal that God is true. …

    John 20:21-23

    Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you. And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost: Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.

Proof texts as given in the original; verse text from the King James Version.