13. Our indulgence ought to extend much farther in tolerating imperfection of conduct. Here there is great danger of falling, and Satan employs all his machinations to ensnare us. For there always have been persons who, imbued with a false persuasion of absolute holiness, as if they had already become a kind of aerial spirits, spurn the society of all in whom they see that something human still remains. Such of old were the Cathari and the Donatists, who were similarly infatuated Such in the present day are some of the Anabaptists, who would be thought to have made superior progress. Others, again, sin in this respect, not so much from that insane pride as from inconsiderate zeal. Seeing that among those to whom the gospel is preached, the fruit produced is not in accordance with the doctrine, they forthwith conclude that there no church exists.
Calvin, Institutes 4.1.13