According to the findings of an excavation in 1948/50, the original building of the church Alt-St. Thomä (slate tower) can still be assumed to date from the Carolingian period. It was a single-nave rectangular hall church. A larger church, the Romanesque basilica, which can still be traced in essential parts in today's construction, was built shortly after 1180 and was first mentioned in a document from 1203. In the period between 1230 and 1300, the church was extended and rebuilt almost without interruption.
The effects of the Soest feud and the Thirty Years' War led to the decay of the church. The congregation no longer had the money to pay for renovation. Instead, it acquired the Minoritenkirche (New St. Thomas) in 1851 and moved. In 1868/69, the Prussian state bought the building, renovated it, and turned it over to the Reformed congregation.
On March 7, 1945, the church was badly hit by bombs. Reconstruction did not begin until 1963, but the interior was only partially repaired. The steeple, which was renewed in 1653 after a lightning strike, is a distinctive feature that is visible from afar in the cityscape. It is strongly inclined to the southwest, or rather crooked. This is why it is popularly known as the "Leaning Tower". For a long time it was assumed that the tower was intentionally built against the west winds. However, an expert opinion from 1984 names rot in the valley beams as the main reason. This clarification, however, is hardly noticed by the public, the old interpretation is more "sympathetic" to the people of Soest.