I find the whole membership/card thing to be interesting consept because I could wip it out in front of some evengelical trying to "save my soul" and show him/her as a form of blasphemy. And it's like a certain percentage of these types would actually liston, truly liston, to anything we are telling them about anything that contradicts anything that thay hold sacred.
So perhaps a statement somewhere on both the certificate and the card to the effect that said recepiant of materials has sold her/his soul to Satan may be helpful in this regard, if not as a form of a Satanic symbolism. This is just my tid bit, my two cents, my suggestion if you will.