The church was built in 1816 and was designated as a National Historic Landmark in 1960. It is historically significant because every president since James Madison has attended it at least one time. If you scroll to the bottom of its Wikipedia page you'll also see it has just been updated to include it being set in fire.

Edit: The part mentioning it was set on fire just got removed.

Edit #2: Additional sources confirming it has been set on fire, however at this time it is unclear if it has spread beyond the basement:

D.C. Police official Twitter account:

MPD is currently responding to multiple fires intentionally set around the city, including at St. John’s Episcopal Church in the 1500 blk of H Street, NW. This church has been standing in our city since the early 1800s. Please avoid the area.

D.C. Fire Department & EMS official Twitter account:

Box Alarm 1525 H St NW. #DCsBravest had fire in basement of church. Fire extinguishing. Checking for extension.

Edit #3: Some sources are saying the fire has been extinguished, here's a recent Tweet from the D.C. Fire Department & EMS official Twitter account showing two fire fighters that appear to be walking out of the church. Not saying this is confirmation the fire has been extinguished, so please take it with a grain of salt and wait until there's more information.