“Last time I checked”
Check again.
Oklahoma constitution: “ Perfect toleration of religious sentiment shall be secured...”
And, at the federal level, there’s that bit about “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof”. The “establishment” part not only refers to the creation of an official state religion, but to whether or not any religion has been “established”.
On top of that, there’s the bit about freedom of speech. If they let one religious group put up a display on the public square, they can’t reasonably refuse another so long as the other group’s display is appropriate.