The liberty framing is crucial. It means that Christian greetings, like other Christian signs and symbols, ought to be permitted to private citizens because the state ought not privilege one religion over any other. This is a classically liberal approach to religion, and it presumes that religion is something that can be safely privatized, domesticated, narrowed to a point of personal preference or, if you’re feeling cheeky, a salutation proffered to a store clerk you don’t know as they pick up working hours over the holidays. Go ahead and say it if you want, or don’t if you don’t; if you get really lucky, somebody you already don’t like may even be bothered by it. This sentiment contains almost every pathology of contemporary American life, but it’s not Christian, and aggressively wishing others a “Merry Christmas” strictly to assert that your in-group is currently empowered isn’t a victory for the faith, even if it passes for one in our current conditions.
On the other hand, if you want to see the measure of a politician’s commitment to Christianity, try telling them something they don’t want to hear. In a Nov. 22 letter to the Senate analyzing the Republicans’ tax reform bill, the United States Conference of Catholic Bishops declared that “Congress must . . . make certain that the nation does not further enshrine indifference toward the poor into law,” a most un-Christian practice, by amending its legislation to “better ensure a just and moral framework for all.” Congress didn’t listen.