The answer to this stuff is to laugh in their face and tell them the Soviet Union is thataway.
Though absent from the Constitution, a phrase close to “separation of church and state”, but used for malevolent purposes and expanded to name education, does appear in Article 52 of the constitution of the Soviet Union (1977): “In the USSR, the church is separated from the state, and the school from the church.” (http://www.departments.bucknell.edu/russian/const/77cons02.html#chap06)