incongruous, in congress, in Congress

Incongruous or incongruous means not fitting or suitable, out of place or out of character --
That old settee looks incongruous in a modern room.
A non-smoking casino bar seems incongruous.

In congress or in congress means [ in ] a meeting or discussion ( congress ) --
They leaned toward one another, and seemed to be in congress.
They remained in congress until they concluded the deal.

In Congress ( always capitalized ) means during a meeting of ( in ) the national legislature of the United States ( the Senate, the House of Representatives, or both ) , or the national legislature of some other country ( Congress ) --
Discussion of the new measure stalled in Congress.
Can you find a news item about our representative's votes in Congress?

Now that you know that, you can say --
"An anarchist serving in Congress is incongruous."