Donald Trump is at it again. This time, the Republican presidential frontrunner suggests that the United States should close the border to all Muslims—including Muslim-Americans traveling abroad. Anyone who cares an iota about religious liberty should denounce this reckless, demagogic rhetoric. Trump, of course, is a master at knowing and seizing a moment. The country is reeling from a terrorist attack by two Islamic radicals. Moreover, the president seems to many to have little plan to eradicate the threat of the Islamic State from building a massive caliphate in the Middle East and exporting terror all over the world. Enter...