The real questions are: What other decorations are in that building, what is the procedure for decorating, who is responsible for decorating, and what are the guidelines for decorating?
If there are other types of decorations there, what is wrong with one of them having a religious theme? Is religious expression not permitted while other varieties of expression is permitted?
If so, then that is viewpoint discrimination
Someday some court will order that all the arms on the crosses at Arlington Cemetary must be removed.