Johnnie is absolutely correct, by preventing the sign from advertising a church or a religion that is prohibiting the free exercise of that religion. Would a business or a non-religious civic organization be allowed to advertise?
Typically in any matter of a minority asking for equality it is not equality that they are looking for, rather special treatment for their group. In the case of religion in our country the atheist is not looking for equality of their religious view, but rather for their view to be the only one allowed to be heard. Now we have religion in school outlawed, which in turn makes atheism the only "religion" protected by law.
The same could be said for hate crime legislation which makes a crime against my children less of a crime because they don't fit a protected ethnic, religious, or sexual preference group. How is it constitutional to say that my children are less important than another? If we truly believe that all men are created equal, equal should mean equality for all.