I'd stick to writing Anabaptist groups. Not sure what you have in Colorado, but generally the Amish are fairly easy targets. And their buggies are easy to catch up with, if you want to toss a few tracts at 'em.... Cautious.
.....that would be "tongue in cheek"......just in case you were wondering. (no offense intended) Here's the serious part:
I would not author letters to secular or pagan organizations (Rome was full of pagans, but Paul never once wrote to them). So that'd exclude any group that did not hold to the gospel of Jesus Christ.
Then I'd narrow that down a bit. I wouldn't write in an attempt to correct a the theology of a church to which I did not belong. Paul wrote to churches, but under apostolic authority....which I am not claiming. If I address a local congregation for teaching false doctrine then I am exercising church discipline. I can't do this if I am not at least a part of that assembly. I certainly can't judge the servant of Another, and that isn't my responsibility.
So the only way I could send the letter, and claim biblical ground, is if it were my church. But this would not be in accordance with biblical discipline. First I'd confront the man face to face. If he doesn't repent then I'd take a brother or two. If this doesn't work then I'd take it before the church. He still doesn't repent then he's removed from the church.
That's the biblical method (but be careful....if you find yourself standing alone then you may well be the one removed....heretics rarely believe themselves heretical). Thumbsup
