more Catholic than the Pope

Adhering more stringently to Roman Catholic practices and doctrine than is required by church doctrine.
(by extension) Adhering more stringently to any norm more strictly than is required by the arbiters of the norm.
- 2005, Jan S. Prybyla, The American Way of Peace: An Interpretation‎:
  The recently liberated European states east of the Oder-Neisse line, seeking not only entry into NATO but membership in the Union club as well, had to be more circumspect, more “European” so to speak, more Catholic than the pope
- 2008, Robert Harvey, The World Crisis: The Way Forward After Iraq‎, page 23:
  It has been asserted that Blair, far from seeking to slow Bush's rush to war, urged him to be tough, in an attempt to be 'more Catholic than the Pope'.
- 2011, Barbara W. Tuchman, Practicing History: Selected Essays‎, page 59:
  Strict disciples of history as a science may scorn the intuitive process, but that attitude comes from being more Catholic than the Pope.

Usually used in a negative sense of someone who is excessively pious or rule-oriented.

adhering more stringently to Roman Catholic practice than is required

English