It's not his courthouse. If Roy Moore wants to buy a building with his own money he can put anything he likes in it. The Alabama State Courthouse does not belong to him.
Judge Moore has the official authority to do it.
I believe you are correct in arguing that there is no issue with Moore's first amendment rights as a citizen.
But I also believe it is entirely appropriate in his official capacity to place a monument to what is undeniably a moral basis of our law.
A: It's not his courthouse.
That's a dodge. Let's nail you down:
Where in our Constitution is Judge Moore prohibited from paying with his own money to put a statue in front of any building?