It seems that a sigil is, most commonly considered, a symbol used in magic. That sets off a loud warning signal in my spirituality, one I don’t want to ignore.
According to several passages in the Bible (Jeremiah 23:13-24; Deuteronomy 13:1-5; 2 Peter 2:1), God had no love for false prophets because they were directly and harmfully affecting the people He created and loved.
I believe, looking at your question, that you are asking who might be the cultural leader of Christians, rather than strictly who is a leader on paper.
My thoughts about the two statements, "I believe in God" and "I know that God exists," could be summed up by two words, ‘ambiguous’ for the first declaration, and ‘apprehension’ for the second.