"To fly the flag" means to proudly display or represent something, usually an organization, a country, or a cause. It can also symbolize showing support or loyalty towards a particular group or idea. Full definition
I think if you have an opportunity to access mainstream media and fly the flag for what we believe in and stand for, generally that's an opportunity to be taken. (premierchristianity.com)
In addition to ships flying flags of convenience, some ships have no nationality at all. (politics.stackexchange.com)
Did you know you can only fly your flag at night if its illuminated? (rocketlawyer.com)