Augustine on Heretics

"FOR WHILE the hot restlessness of heretics stirs questions about many articles of the Catholic faith, the necessity of defending them forces us both to investigate them more accurately, to understand them more clearly, and to proclaim them more earnestly."

~St. Augustine: The City of God, Bk. 16, Chap. 2. (5th cent.)

Isidore of Seville on Heresy

"THEREFORE, heresy is so called from the Greek word meaning 'choice', by which each chooses according to his own will what he pleases to teach or believe. But we are not permitted to believe whatever we choose, nor to choose whatever someone else has believed. We have the apostles of God as authorities, who did not themselves of their own will choose what they would believe, but faithfully transmitted to the nations the teaching received from Christ. So, even if an angel from heaven should preach otherwise, he shall be called anathema."

~St. Isidore of Seville: Etymologies, 8, 3. (7th cent.)

Cyprian on Heresy

"WHOEVER has been separated from the Church is yoked with an adulteress, is separated from the promises made to the Church. Nor shall he who leaves Christ's arrive at Christ's rewards. He is a stranger, he is sacrilegious, he is an enemy. Who has not the Church for mother can no longer have God for father."

–On the Unity of the Catholic Church, 6.

"NEVERTHELESS, the Lord allows and suffers these things to be, while each man's will free, so that while our hearts and minds are tested in the crucible of truth, the sound faith of those that are approved may shine forth clear and undimmed."

–On the Unity of the Catholic Church, 10.

Read more here