I can't. And I can't show you a "fair trial" clause, either. Or "religious freedom" or "right to an attorney if you can't afford one."
If you accept that the courts have to apply the Constitution to the facts, then you'll understand why they had to rule on what the Constitution meant in terms of the First Amendment.
That doesn't mean that you have to agree with their rulings, or that they can't change.
But if you don't think that the court has the right to interpret the Constitution, then you're left with the uncomfortable position of everyone being equally able to decide what it means, which means that it means close to nothing.
Who decides?
But it does mean that you must follow those rulings until they are changed.