Regardless of your religious beliefs, when visiting Washington D.C., you should go to the Basilica of the National Shrine of the Immaculate Conception. It is the biggest Catholic Church in the United States. It has a modern architecture compared to the ones you can find in Europe or Latin America.
On top of the main altar, below the dome, there is a statue of Jesus Christ that appears to be angry. The main church has many chapels, and it is one of the most beautiful built for Our Lady of Guadalupe. On the ground floor, the basilica has a small museum, a church, and many chapels. Next time you go to Washington, make sure you visit it. It is about four miles from the most famous monuments.
Here are some pictures: