Ignatius of Antioch

Wherefore, as children of light and truth, avoid the dividing of your unity, and the wicked doctrine of the heretics, from whom "a defiling influence has gone forth into all the earth." But where the shepherd is, there do ye as sheep follow. For there are many wolves in sheep's clothing, who, by means of a pernicious pleasure, carry captive those that are running towards God; but in your unity they shall have no place.

Epistle of Ignatius to the Philadelphians ↗