The secularisation of the domains of the Teutonic Order in Prussia led to the establishment of the first Lutheran territorial church in the world. This fact is almost forgotten today, and this is evident even in specialised literature on the Reformation. The article outlines the introduction of the Reformation in Prussia, considering it as an example of its smooth and successful entrenchment. In order to show this, the late stage of the rule of the Teutonic Order is defined, showing that fundamental reform was triggered by a multi-layered crisis characteristic of the Order’s domains in Prussia. The article shows that, in coordination with Martin Luther and Philipp Melanchton, and assisted by his bishops, after becoming the first Duke of Prussia in 1525, Albert, the Grand Master of the Teutonic Order, implemented reforms in his domains that resembled the main problems raised by the Reformation in an almost exemplary way. But at the same time, it shows that the introduction of the Reformation in Prussia was not a unidirectional process, for Duke Albert supported Andreas Osiander’s ideas for some time, before he gradually entered the ranks of the confessors of Augsburg.