In the matter of the bus shelter ads, government was not itself putting up Christian messages, it was interfering with the ability of a private organization to do so. Sometimes holiday gifts are challenging. Many a parent will struggle with the assembly instructions for a bicycle. Grandma's fruitcake is a trial for both our dental work and our tactfulness. America's gift from the Founding Fathers is no different.
Religious liberty requires thoughtfulness, tolerance, and vigilance.
That way it will last for another two centuries or more.