Arrested and imprisoned at Brussels for this cause, Enzinas received while under duress visits from four hundred citizens of that city who were Protestants.

To control the book trade an oath was exacted of every bookseller [Sidenote: 1546] not to deal in heretical works and the first "Index of prohibited books," drawn up by the University of Louvain, was issued.

A censorship of plays was also attempted.

This was followed by an edict of 1550 requiring of every person entering the Netherlands a certificate of Catholic belief.
As Brabant and Antwerp repudiated a law that would have ruined their trade, it remained, in fact, a dead letter.
Charles's policy of repression had been on the whole a failure, due partly to the cosmopolitan culture of the Netherlands and their commercial position making them open to the importation of ideas as of merchandise from all Europe.

It was due in part to the local jealousies and privileges of the separate provinces, and in part to the strength of certain nobles and cities.