The ''Declaratio Ferdinandei'' ( en, Declaration of Ferdinand) was a clause in the Peace of Augsburg, signed in 1555 to end conflicts between

Catholics The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptized Catholics worldwide . It is among the world's oldest and largest international institutions, and has played a ...

and

Protestants Protestantism is a Christian denomination, branch of Christianity that follows the theological tenets of the Reformation, Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century agai ...

within the

Holy Roman Empire The Holy Roman Empire was a political entity in Western, Central, and Southern Europe that developed during the Early Middle Ages and continued until its dissolution in 1806 during the Napoleonic Wars. From the accession of Otto I in 962 unt ...

. The Peace created the principle of ''

Cuius regio, eius religio () is a Latin phrase which literally means "whose realm, their religion" – meaning that the religion of the ruler was to dictate the religion of those ruled. This legal principle marked a major development in the collective (if not individual ...

'' (Latin for "''whose realm, his religion''"), which meant that the religion of the ruler decided the religion of the inhabitants. The ''Declaratio Ferdinandei'' exempted knights and some of the cities under the jurisdiction of an ecclesiastical prince if they had practiced

Lutheranism Lutheranism is one of the largest branches of Protestantism, identifying primarily with the theology of Martin Luther, the 16th-century German monk and Protestant Reformers, reformer whose efforts to reform the theology and practice of the Cathol ...

for some time (Lutheranism was the only branch of Protestantism recognized under the Peace). The provision was not publicized as part of the treaty, and was kept secret for almost two decades. After Catholic victories early in the

Thirty Years' War The Thirty Years' War was one of the longest and most destructive conflicts in European history, lasting from 1618 to 1648. Fought primarily in Central Europe, an estimated 4.5 to 8 million soldiers and civilians died as a result of batt ...

, the ''Declaratio Ferdinandei'' was overturned in the Edict of Restitution of 1629, which was part of Ferdinand II's master plan to reconvert the Holy Roman Empire to Catholicism.Parker, pp. 87–88 The overturning of the ''Declaratio Ferdinandei'' and other religious persecution helped rekindle the Thirty Years' War, changing it from an internal conflict within the Holy Roman Empire into an international religious war.

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See also