The Gedächtniskirche — Memorial Church of the Protestation — was built between 1893 and 1904 to mark a specific moment in 1529 when history turned. At the Diet of Speyer that year, the Holy Roman Emperor's Catholic majority voted to roll back earlier concessions to Lutheran worship. A group of Lutheran princes formally objected — they protested — and in doing so gave an entire religious movement the word it still uses. The church stands on the approximate site of that confrontation.
The building itself is neo-Gothic, built at a time when German Protestantism was keen to make its history visible in stone. The interior is lighter than you might expect: tall windows, pale stone, and stained glass that is good without being exceptional. It is less a working parish church than a monument, and it reads that way — earnest, considered, and a little ceremonial. Worth a look if you are interested in Reformation history, which is woven through the whole of Speyer: the city hosted multiple Imperial Diets and was a major stage for the early decades of the Protestant-Catholic conflict.