Tell your boss you've been gravely stumbled by them contacting him and (by implication) threatening your employment. Tell him that upset you so much, you refuse to talk to them. Then tell him you're consulting a labor attorney. He'll get the message that if he gives you crap about your religious affiliation as an employer he's liable. Then do it!
Good idea, but before you do it, make sure you DOCUMENT everything, even if you just jot down notes. Make sure you have dates and times. Keep all messages, letters, etc. Oh and start looking for another job. It ain't gonna turn out good no matter what you do...