Three marks are usually enumerated: the preaching of the Word, the administration of the sacraments, and church discipline.
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.The Anglican Thirty-nine Articles states in chapter 19: The visible Church of Christ is a congregation of faithful men, in which the pure Word of God is preached, and the Sacraments be duly ministered according to Christ’s ordinance, in all those things that of necessity are requisite to the same.
[2]Louis Berkhof notes that Reformed theologians have differed as to the number of marks: Theodore Beza spoke of only one (preaching), John Calvin and Heinrich Bullinger spoke of two (preaching and sacraments), while Peter Martyr and Zacharias Ursinus spoke of three – preaching, sacraments and discipline.
[3] Nevertheless, Edmund Clowney points out that Calvin "included discipline in the proper observance of the sacraments.