Change the last bit from "don't look a gift horse in the mouth" to something like "be willing to accept this with a spirit of grace, understanding the shared concern for your cause."
If it were in the middle of the letter it would be one thing, but I think ending on a saying like that might weaken the letter, literarily speaking.
Also, maybe take this: "First of all, I believe you have demonstrated a narrow-minded attitude to role playing games. They are not gateway games to Paganism or Satanic Worship. "
And say something like this: "First of all, I your words would seem to demonstrate a narrow-minded attitude to role playing games. If that was not your intent, it is nonetheless the attitude many will believe you are displaying. These games are not gateways to Paganism or Satanic Worship. They have no inherent world-view attached to them, being intended solely for tools for enjoyment and imagination." (Or something to that affect; show an effort to try to reach common understanding; it will go over much better, I daresay.)
Overall, a very calm-sounding letter. My changes are minor, and more of a "how I would write it" than anything.
I think that it's a very good response, especially as an attempt to actually get them to listen. Bile and anger only breed more reactionism.