Because of the influx of Masovians into the southern lakeland, the area started to be known as "Masuria" from the 18th century. During the Protestant Reformation the Masurians, like most inhabitants of Ducal Prussia, became Lutheran Protestant, while the neighboring Masovians remained Roman Catholic. In 1525, the Duchy of Prussia, a Polish fief until 1657, was founded from the secularized order's territory and became the first ever officially Protestant state. The small minority of Protestant Masovians in southern Catholic Masovia inside Poland emigrated later to Prussian Masuria. Masuria became part of the Kingdom of Prussia at the Kingdom's founding in 1701, and part of the Prussian-led German Empire at the Empire's founding in 1871.