Day of the devil. - what is the sign of excommunication in the church?

Can anyone enlighten me as to what the sign of excommunication is that has frightened Richard Griffith's character when he first encounters Keith Allen's character? Try as I might, I cannot see anything on the wall, where I guess the sign, which Allen's character says is only a trick shadow, is supposed to be! Thank you to anyone who can see it and tell me what it is!