Haeresis (the word from which heresy is derived), in the ancient pagan world, appears to have meant a school of thought. The tolerant attitude of ancient pagans is summed up by Quintus Aurelius Symmachus (a hero of mine):
Everything is full of gods. Whatever men worship, it may fairly be called one and the same. We all look up to the same stars; the same heaven is above us all; the same universe surrounds every one of us. What does it matter by what system of knowledge each one of us seeks the truth? It is not by one single path that we attain to so great a secret.
- Quintus Aurelius Symmachus