Let's temporarily substitute "A legislative body" for "Congress" to make this apply to the states. Now, how does Moore, a state supreme court justice, erecting a monument that has the 10 commandmnets on it violate this clause? Logically, how does it even fit? Show me the legal opinion that applys this clause, standing alone, as it logically must, to the action of a judicial officer. The "lemon test" presuposes a fit.
Do I hear you saying that anything the Supreme Court says regarding the Constitution stands, in spite of any reasonable interpretation thereof? You agreee with Roe v Wade, and the second legal tender case, I take it? You factor not the packing of the court by a president and a compliant Congress to establish a socialism into your opinon?
There is no rule of law when the written law can be preverted as a govenment body so wishes. There is only the rule of man, which the People ordained and established this country for the precise purpose of avoiding.
Your lack of knowledge of our form of government scares the piss out of me. If you want rule by a king or an asistocracy then why don't you move to a country that has one or the other? 