What Are the Marks of the True Church?

The Belgic Confession
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.
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So it's only the true Church if the preaching of the Gospel and administration of sacraments are pure, and church discipline is used for correcting faults. Calvin only believed in two marks of the Church: “the preaching of the word and the observance of the sacraments.” He argued that to lose these two marks would “destroy the true and genuine distinction of the Church”:​

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