This Essay evaluates the theory of popular constitutionalism by exploring the concept of constitutional fidelity and the practical requirements it imposes on the exercise of interpretive authority within constitutional democracies. Popular constitutionalists argue that the people ought to play a greater role in the process of constitutional interpretation, and advocate for reforms that would make this command a reality. Popular constitutionalism’s opponents reject such reforms on the ground that final interpretive authority over the Constitution lies properly with the justices of the U.S. Supreme Court. Neither side of the debate has devoted much effort to uncovering what the people know about interpreting the Constitution or the implications their level of knowledge has for the theory of popular constitutionalism. This Essay’s inquiry into constitutional fidelity reveals two important reasons why they should: (1) all who exercise interpretive authority, including the people, must faithfully exercise that authority; and (2) the ability of any interpreter to faithfully interpret the Constitution depends upon her acquisition of particular knowledge and reasoning-based competencies. The Essay identifies the essential content of these competencies, and then considers the extent to which the people and the justices possess them. The empirical evaluation conducted suggests that the people lack these competencies and the justices possess them. The Essay concludes by explaining why this finding justifies rejecting popular constitutionalist proposals to delegate interpretive authority to the people.