"
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"
respecting = to have reference to
establishment = a settled arrangement; especially : a code of laws
My interpretation of this passage is that Congress is not allowed to make a law to establish an official state religion. Having the commandments there, regardless of this man's goals in doing so, is fully within his rights (assuming he has the right to place a display of any type there). It in no way establishes a state religion, nor does it prohibit the right of citizens to freely practice their religion of choice. It simply acknowledges the origin of United States law. His goals are of no consequence here.
This is not a first amendment issue, it is an issue of if he had the right to put anything there without permission in the first place. The few stories I've read say that he had it installed after the building closed, keeping the other justices in the dark. If he had the right to do this, then in my opinion he had the right to do it period.
Of course, this whole argument is based on
my personal interpretation of the words in the first amendment, and I'm sure you'll let me know if there is a flaw in it.
An interesting side note: Ever been to a courthouse and gazed upon the statue of
Themis, the Greek goddess of Justice? The blindfolded figure holding the scales of justice is a Greek goddess, a religious figure.