Prussian Union of Churches
The Prussian Union of Churches (known under multiple other names) was a major Protestant church body which emerged in 1817 from a series of decrees by Frederick William III of Prussia that united both Lutheran and Reformed denominations in Prussia. Although not the first of its kind, the Prussian Union was the first to occur in a major German state.
Source: Wikipedia — Prussian Union of Churches (CC BY-SA 4.0)