To what character (or characters) does the title, "Defender of the Faith" refer? Is it used straightforwardly or ironically?

The title, Defender of the Faith, seems to refer to Sergeant Marx.  Sergeant Marx often finds himself in situations where he is forced to defend his Jewish faith.  The title is ironic because Sgt. Marx does not choose this role; he is forced into it by his subordinates.  The young men in the story are constantly asking for favors of Marx simply because they know that he is Jewish, like them.  Whenever Marx tries to refuse a request, he is accused either implicitly or explicitly of not being a “true” Jew.  The irony in this is that Marx does not only have to defend his faith from the gentiles, but also from his fellow followers of Judaism.