You may have noticed that when President Trump got elected, millions of Americans took to the streets saying that Trump was "UnAmerican" and was "Not Their President." They were not suggesting that Trump was not an American citizen (he is) or that he wasn't literally the president (he also is), but rather, they were suggesting that his rhetoric and policy suggestions were antithetical to what it means to be an American. They were attempting to announce boundaries as to what it means to be American (beyond just having citizenship). Certainly there's a lot of wiggle-room in what it means to be an American, but most would probably agree some things are way out of bounds (say, communism or a racially-based dictatorship).

So, when one says someone else is "not Christian," they may not mean that the other party doesn't literally believe in Jesus Christ, bur rather that the other party is stepping outside of Christian normative ideals or understandings, especially in relation to theological or soteriological matters. If we lack these boundaries in discussions and understanding of the Christian, then anything is totally up for grabs, and the Gospel loses all meaning, much as what it means to be an American would lose all meaning if we abandoned discussion on what American values are or should be.I would personally never call someone "not Christian" unless they make some outright heretical remarks (think about someone like Bsp. Shelby Spong, or sedevacantists).