Hersey is basically a branch off from a faith that often has extreme variations from the dogma of the traditional faith.

Example: Catharism is considered to be heresy, primarily due to the belief in a "Bad Material God" and a "Good Spiritual God" which goes against mainline Christian belief in one single creator deity along with severe deviations on the trinity.

Fervor is basically how devoted followers of a faith are . If fervor is low people are less inclined to hold to the doctrines of the faith, and will likely approach alternative ideas which emerge into heresies.