It's no different from when a legislature passes an unconstitutional law. We are not bound to obey it, simply on their sayso. The law has to be constitutional before we are bound by it. The same goes for court injunctions. Otherwise, courts could amend the Constitution at whim.
There is no such "fact", only Judge Moore's opinion. While he is certainly entitled to that much, the fact is that we have a procedure for determining the constitutionality of some thing or another, and that process has determined that his display violates the Constitution. Now Judge Moore would simply like his personal opinion of the Constitution to override the legal authority of those who have the duty to make such rulings.
And if you let him, Roy Moore will have successfully carved out another privilege that I highly doubt he is willing to extend to those who come before him in the courtroom - the privilege of determining for oneself which court rulings one does and does not have to obey. A nice gig if you can get it, but I am rather skeptical that Judge Moore would look kindly upon me if I baldly applied what he is doing for himself to my own case as he pronounced a ruling in it. "Sorry Judge - your ruling is unconstitutional, so I'm not going to obey it."
Or perhaps not. How much luck do you think I would have in front of Judge Moore with that one?