2 The bold and shameless heresy of the Arians, after being publicly cut off from the body of the Church, still abides in its own error, and does not do us much harm because its impiety is notorious to all. Nevertheless men clad in sheep's clothing, and presenting a mild and amiable appearance, but within unsparingly ravaging Christ's flocks, find it easy to do hurt to the simpler ones, because they came out from us. It is these who are grievous and hard to guard against.
It is these that we implore your diligence to denounce publicly to all the Churches of the East; to the end that they may either turn to the right way and join with us in genuine alliance, or, if they abide in their perversity, may keep their mischief to themselves alone, and be unable to communicate their own plague to their neighbours by unguarded communion. I am constrained to mention them by name, in order that you may yourselves recognise those who are stirring up disturbance here, and may make them known to our Churches.
My own words are suspected by most men, as though I had an ill will towards them on account of some private quarrel. You, however, have all the more credit with the people, in proportion to the distance that separates your home from theirs, besides the fact that you are gifted with God's grace to help those who are distressed. If more of you concur in uttering the same opinions, it is clear that the number of those who have expressed them will make it impossible to oppose their acceptance.
Source: Letters (New Advent)