What legal argument can be made to say that Christians have the right to display religious symbols in public places?
From a legal point of view, the best way to make this argument is to say that, when Christians display religious symbols in public places, it does not amount to a government endorsement of those symbols.
The Establishment Clause (as it is understood today) forbids the government from promoting a given religion. But it does not forbid individuals from expressing their religious beliefs. The argument you should make is that displaying religious symbols in public places is simply a statement of personal belief. Since it is just a statement of personal belief, it should be legal under the First Amendment.