Page "Dutch Republic" Paragraph 23
In the Union of Utrecht of 20 January 1579, Holland and Zeeland were granted the right to accept only one religion ( in practice Calvinism ).
Every other province had the freedom to regulate the religious question as it wished, although the Union stated every person should be free in the choice of his personal religion and no person should be prosecuted based on his or her religious choice.
William of Orange had been a strong supporter of public as well as personal freedom of religion and hoped to unite Protestants and Catholics in the new union, and for him the Union was a defeat.
In practice, Catholic services in all provinces were quickly forbidden and the Reformed Church became the " public " or " privileged " church in the Republic.