The True Church, according to the Belgic Confession, can be recognized by three essential marks. 1. It practices the true preaching of the gospel; 2. It maintains the pure administration of the sacraments (baptism and the Lord's Supper) and 3. It exercises church discipline for correcting and punishing sins.
In the second mark, the administration of baptism and the Lord's supper serve as visible, tangible signs, appointed by God, to more fully declare the promise of the gospel. They are visible signs and seals of a spiritual reality.
The first and third marks, the preaching of the gospel and church discipline, are what we call "keys of the kingdom" by which the church is given the authority to admit to, or bar from the kingdom of heaven. Ultimately, eternal life and eternal condemnation of individuals are in the hands of God alone, but He has commissioned the church as His instrument in administering the kingdom of heaven on earth according to the Word of God.
The Heidelberg Catechism states what we believe the Bible teaches regarding how the kingdom of heaven is opened or shut by the preaching of the gospel:
Q: How is the kingdom of heaven opened and
shut by the preaching of the Holy Gospel?
A: In this way: that, according to the command of
Christ, it is proclaimed and openly witnessed to
believers, one and all, that as often as they accept
with true faith the promise of the Gospel, all their
sins are really forgiven them of God for the sake of
Christ’s merits; and on the contrary, to all
unbelievers and hypocrites, that the wrath of God
and eternal condemnation abide on them so long as
they are not converted. According to this testimony
of the Gospel, God will judge men both in this life
and in that which is to come.__Q. 84
Matt. 16:19; Mark 16:15-16; John 20:23