St. Clement of Rome is a beautiful Modernist church out west in Des Peres; I was able to photograph its exterior recently. Rob Powers has stunning pictures of the interior here. The church of San Clemente in Rome is one of my favorites; it is built on the remains of a Paleo-Christian sanctuary, which is itself built on the remains of earlier Roman streets, including a well-preserved Mithraeum. It is well worth the visit if you are in Rome, as it is only a ten minute walk from the Colosseum.