TBC: Error has been with the Church since the beginning, and this ancient admonition remains practical for today.
"Error, indeed, is never set forth in its naked deformity, lest, being thus exposed, it should at once be detected. But it is craftily decked out in an attractive dress, so as, by its outward form, to make it appear to the inexperienced (ridiculous as the expression may seem) more true than the truth itself. One far superior to me has well said, in reference to this point, "A clever imitation in glass casts contempt, as it were, on that precious jewel the emerald (which is most highly esteemed by some), unless it come under the eye of one able to test and expose the counterfeit. Or, again, what inexperienced person can with ease detect the presence of brass when it has been mixed up with silver?" Lest, therefore, through my neglect, some should be carried off, even as sheep are by wolves, while they perceive not the true character of these men,-- because they outwardly are covered with sheep's clothing (against whom the Lord has enjoined us to be on our guard), and because their language resembles ours, while their sentiments are very different" (Irenaeus, "Against Heresies," Electronic Bible Society, CD Rom, ECF, pg. 345).