Strictly speaking, it may be said that the true preaching of the Word and its recognition as the standard of doctrine and life, is the one mark of the Church. Without it there is no Church, and it determines the right administration of the sacraments and the faithful exercise of Church discipline. Nevertheless, the right administration of the sacraments is also a real mark of the Church. And though the exercise of discipline may not be peculiar to the Church, that is, is not found in it exclusively, yet it is absolutely essential to the purity of the Church.
Louis Berkhof