This has been a week of surprises from Chief Justice John Roberts. Wednesday he joined the Court’s more liberal members in a 5-4 decision in a significant First Amendment case. Tuesday at the argument in Obergefell v. Hodges, the marriage equality cases, he suggested that a ban on same-sex marriage might present a “straightforward question of sexual discrimination.” These superficially surprising moves support Roberts’s oft-repeated claim that he believes the Justices should decide cases based on the law, not politics. But as Roberts’s tenth term as Chief Justice comes to a close, the biggest tests of that claim are still...