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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 personal religion and that no person should be prosecuted based on religious choice. William of Orange had been a strong supporter of public and 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.

What granted Holland and Zeeland the right to accept only one religion?
the Union of Utrecht of 20 January 1579