Chris and Lars are both right. Faith cannot be private. "Private only" faith is no faith at all. It negates the Great Commission and avoids the conflict that Jesus promised we would (and should) experience. But faith starts in the inner sanctum, in the quiet places of our relationship with Jesus Christ. That informs our public behavior. Or should.

Romans 10:10 - Believe in your heart (private) and confess with your mouth (public).

Christians meet Jesus in private, but we are required to live our faith out loud. If anyone but the casual passerby is uncertain of your spiritual status, you're not living up to God's standard, but angrily waving signs in the street is not evangelizing for Christ. So maybe the Satanist's prayer had its desired effect.