Your assumption that you understand the Establishment Clause better than two centuries of jurists, is laughable.
When a government official, paid by the People, practices a religion in a public meeting (paid for by the People), he/she is favoring that religion over all others as a representative of the government. This is entirely different from practicing at home, in the bathroom, in the trunk of a car, or wherever you right-wing nutjobs get your rocks off.
Now try really hard to imagine (you can google that term) that your Mayor is consistently opening up planning sessions by quoting the Quran and praying to Mecca on a prayer rug. Every time. Shall I describe the scene for you again, or is your brain working now?