Saint Isaac's Cathedral a.k.a. Isaakie vskiy Sobor was completed in 1858.
In 1931, it was stripped of it's religious ties and turned over to the Museum of the History of Religion and Atheism.
Six years later, the building was named the museum of the cathedral, and, as you can see, it's covered in art.
Now worship services are once again held in part of the building.