Why study heresy? Isn't heresy something better left unsaid? A dirty little secret of the church? Unnecessary conflict over minor details?
In truth, heresy has proved a great benefit to the church. By requiring an organized effort to combat it, heresy has helped to consolidate and codify orthodox beliefs.
Also, heresies return in slightly altered forms. Knowledge of their existance, and the proofs against them are of great use to all.
One of the oldest heresies is Marcionism (circa 140). Marcion created a canonical Bible without the Old Testament. He also severely edited the New Testament books to eliminate traces of Judaism. He claimed the loving God described in the New Testament was incompatible with the wrathful God of the Old Testament.
The response to Marcionism was the adoption of the canonical Bible as we know it today. This was not something made up to serve a political purpose, but simply the documenting and formal processing of books that had been held as special ever since they had been written.
Marcionism is still alive and well today. I started, but never finished, the book "Jesus Against Christianity" (Jack Nelson-Pallmeyer, 2001). This book begins with nearly the same argument. Perhaps one day I will read it entirely and can comment more.