A letter of Athansius’ written to a younger disciple concerning how he should handle those who follow false teachers:
I thank the Lord who has given to you to believe in Him, that you too may have eternal life with the saints. But because there are certain persons who, while they affirm that they do not hold with Arius, yet compromise themselves and worship with his party, I have been compelled to write at once…For when any see you, the faithful in Christ, associate and communicate with such people, certainly they will think it a matter of indifference and will fall into the mire of irreligion. Lest, then, this should happen, be pleased beloved to shun those who hold the impiety of Arius. We are specially bound to fly from the communion of men whose opinions we hold in execration. If then any come to you, and, as blessed John says, brings with him right doctrine, say to him, All hail, and recive such an one as a brother. But if any pretend that he confesses the right faith, but appear to communicate with those others, exhort him to abstain from such communion, and if he promise to do so, treat him as a brother, but if he persist in a contentious spirit, him avoid.
Athanasius, Letters of Athanasius, Nicene and Post Nicene Fathers, vol. 4, p. 464.