Any religion would loathe heretics far more than any other enemy. Traditionalists would feel that the explanations and new inventions of heretics could cause the religion to lose its purity and tradition, needlessly confusing loyal believers.
An analogy might be the most terrifying disease on Earth: cancer. Ordinary cells mutate and spread at a terrifying rate. Once the alterations within the body reach a certain extent, survival becomes impossible. Those who have fought and overcome cancer have endured great strain on their bodies.
In this world of True Gods, heretics have different understandings and explanations of the teachings of the True Gods. Such differences can lead to changes in a Divine Concept or in people's beliefs.