And that in itself is against the First Amendment.
a) By banning what it considers "religious displays" (a term that gets more and more vague with every passing year) Congress is "...prohibiting the free exercise (of religion/religious holiday) thereof;..."
b) It can -- and has been -- argued that by banning what it considers "religious displays" Congress is, in fact, "..making laws respecting the establishment of (a) religion..." The religion* being, of course, atheism.
* The definition of the word "religion", while chiefly, is not always Theological in nature or meaning. The word can also mean "a cause, principle, or activity pursued with zeal or conscientious devotion." To many Marxism is a religion.