“The true church can be recognized if it has the following marks: The church engages in the pure preaching of the gospel; it makes use of the pure administration of the sacraments as Christ instituted them; it practices church discipline for correcting faults. In short, it governs itself according to the pure Word of God, rejecting all things contrary to it and holding Jesus Christ as the only Head. By these marks one can be assured of recognizing the true church – and no one ought to be separated from it.
As for those who can belong to the church, we can recognize them by the distinguishing marks of Christians: namely by faith, and by their fleeing from sin and pursuing righteousness, once they have received the one and only Savior, Jesus Christ. They love the true God and their neighbors, without turning to the right or left, and they crucify the flesh and its works. Though great weakness remains in them, they fight against it by the Spirit all the days of their lives, appealing constantly to the blood, suffering, death, and obedience of the Lord Jesus, in whom they have forgiveness of their sins, through faith in him.
As for the false church, it assigns more authority to itself and its ordinances than to the Word of God; it does not want to subject itself to the yoke of Christ; it does not administer the sacraments as Christ commanded in his Word; it rather adds to them or subtracts from them as it pleases; it bases itself on men, more than on Jesus Christ; it persecutes those who live holy lives according to the Word of God and who rebuke it for its faults, greed, and idolatry. These two churches are easy to recognize and thus to distinguish from each other.”
-from the Belgic Confession, Article 29, on “The Marks of the True Church,” 1561