Father of the Church on Avoiding Heretics

St. Ignatius of Antioch Father of the Church on Avoiding Heretics

Chapter 4. Beware of these heretics

I give you these instructions, beloved, assured that you also hold the same opinions [as I do]. But I guard you beforehand from those beasts in the shape of men, whom you must not only not receive, but, if it be possible, not even meet with; only you must pray to God for them, if by any means they may be brought to repentance, which, however, will be very difficult. Yet Jesus Christ, who is our true life, has the power of [effecting] this. But if these things were done by our Lord only in appearance, then am I also only in appearance bound. And why have I also surrendered myself to death, to fire, to the sword, to the wild beasts? But, [in fact,] he who is near to the sword is near to God; he that is among the wild beasts is in company with God; provided only he be so in the name of Jesus Christ. I undergo all these things that I may suffer together with Him, Romans 8:17 He who became a perfect man inwardly strengthening me. Philippians 4:13 

 
The Epistle of Ignatius to the Smyrnaeans

http://www.newadvent.org/fathers/0109.htm

Titus 3:10
A man that is a heretic, after the first and second admonition, avoid

 Please, Don't call Protestants Christians