In January 2002, prominent Catholics from around the world gathered in Rome to celebrate the Spanish priest who founded one of the church’s most conservative and devout groups, Opus Dei.

The event drew cardinals, bishops and other powerful Vatican officials. And among those invited to speak was a future presidential candidate: Rick Santorum, whose faith had become so essential to his politics that on federal documents he listed the trip, paid for by an Opus Dei foundation, as part of his official duties as a U.S. senator from Pennsylvania.