That was a crime in Mexico in the early 1990s?
Yes, it was an archaic law in Australia at that time also, a left over from colonial days of Catholic persecution by the Protestants, but was never enforced here, although it was certainly used by some priests as a 'pretext' for travelling in the clergyman.
Here is an article I found from the Washington Post from 1991, which would have been the year after Father's imprisonment I believe:
By Edward Cody
November 2, 1991
MEXICO CITY, NOV. 1 -- President Carlos Salinas de Gortari announced today he will bestow a new legal status on the Roman Catholic Church in Mexico as a first step toward erasing the country's bitter legacy of official anti-clericalism.
The measure, revealed in an annual report to the nation, formalized Salinas's intention to depart from rigid constitutional restrictions on church activities imposed at the end of Mexico's turn-of-the-century revolutionary chaos, during which land-owning clergy frequently took sides against popular rebellions to which the ruling Institutional Revolutionary Party (PRI) traces its origin.
As the years passed, authorities largely have ignored the legal restraints, and, in a nation that is 90 percent Catholic, church activities are an important part of daily life. Nevertheless, the church's legal status has endured as a passionate issue for many PRI leaders imbued with legends of the ruling party's revolutionary past.