Our most frequently asked question, you'll find a dozen answers to it with a search in this very forum.
The envelope does not say "Church," "Satan," "CoS," or depict any Satanic emblems (or any nonSatanic emblems for that matter). It's mailed in a plain white envelope of no particular note.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."