Everyone looks visually different, and you will encounter, in your own country, a fellow countryman, or woman, whose parents are born in your country, who have the same nationality and legal status as yourself, but a different ancestry, maybe from India, that person will then, in your mind, not be your peer, simply because they look different? Even though you share culture and history and upbringing?
That attitude will be harder and harder to defend, as more and more phenotypes enter your society.
You may even find that your neighbor, who you've known your whole life, actually does not have any religion, or a different one, even though his ancestry goes back forever as natives of your geographical area, then you will no longer count him as your countryman?
So by following this train of logic, and remembering at each point that there really is no such thing as a true Scotsman, the logical reduction of this is, that what makes someone a citizen of your country is entirely and only defined by whether or not they hold a passport from your country, or whatever national identity identifier your country has.
Now, fast forward some generations of actually educating children, instead of indoctrinating them to think they're better or more right than others, but that people are people, and planet earth is home to people, and you end up with a society such as your current one (without the religion and racism), but one that entails the entire planet.
Show me a country, any country, and I will show you country.numberOfCitizens indiduals who actually don't agree on as much as they fool themselves into believing that they do, and yet somehow manage to find some mutual respect and regard for oneanother. There's no good (future) reason to base that regard on citizenship rather than on being human.