We have stated that the marks by which the Church is to be distinguished, are, the preaching of the word, and the administration of the sacraments. For these can no where exist without bringing forth fruit, and being prospered with the blessing of God. I assert not that wherever the word is preached, the good effects of it immediately appear; but that it is never received so as to obtain a permanent establishment, except in order that it may be efficacious. However this may be, where the word is heard with reverence, and the sacraments are not neglected, there we discover, while that is the case, an appearance of the Church, which is liable to no suspicion or uncertainty, of which no one can safely despise the authority, or reject the admonitions, or resist the counsels, or slight the censures, much less separate from it and break up its unity. For so highly does the Lord esteem the communion of his Church, that he considers every one as a traitor and apostate from religion, who perversely withdraws himself from any Christian society which preserves the true ministry of the word and sacraments. He commends the authority of the Church, in such a manner as to account every violation of it an infringement of his own. For it is not a trivial circumstance, that the Church is called “the house of God, the pillar and ground of truth.” For in these words Paul signifies that in order to keep the truth of God from being lost in the world, the Church is its faithful guardian; because it has been the will of God, by the ministry of the Church, to preserve the pure preaching of his word, and to manifest himself as our affectionate Father, while he nourishes us with spiritual food, and provides all things conducive to our salvation. Nor is it small praise, that the Church is chosen and separated by Christ to be his spouse, “not having spot or wrinkle,” to be “his body, the fulness of him that filleth all in all.” Hence it follows, that a departure from the Church is a renunciation of God and Christ. And such a criminal dissention is so much the more to be avoided; because while we endeavour, as far as lies in our power, to destroy the truth of God, we deserve to be crushed with the most powerful thunders of his wrath. Nor is it possible to imagine a more atrocious crime, than that sacrilegious perfidy, which violates the conjugal relation that the only begotten Son of God has condescended to form with us.
(Calvin, John, Institutes of the Christian Religion. Book IV, Chapter 1, Section 10)