I often see people accused of being racist in public, usually because they’ve just said something horrifically bigoted, and one of the go-to defenses (behind “but I have X friends”) is “X isn’t a race:”

For example:
“Mexican isn’t a race,” (because it’s an ethnicity.)
“Islam isn’t a race,” (because it’s a religion.)
“Jewish isn’t a race,” (because it’s a religion and an ethnicity?)

This is not a defense you want to use to establish your “not a racist” bona fides (which is itself a problematic idea, and one I’ll try to address at another time.) That’s because it’s really no defense at all.

One problem with this defense is that it’s ironically incriminating. If you’re saying that X isn’t a race, you’re implying one of two things: either there are such things as races (and X just isn’t one) or that there’s no such things as races, and so no group can qualify. Each of these is problematic for its own unique reason.

If you think there are such things as races, you are a racist, and we don’t need to go any further down that line. The concept of race has no taxonomic validity. There are no distinct populations of humans, and so we all belong to the same species and the same subspecies (that is, race.) What we think of as “race” is entirely socially constructed, its distinctions wholly arbitrary. While it is important in real-world terms, that’s really only because of racism. If you believe in it, you’re part of the problem.

If you think there aren’t such things as races, you could just as validly claim that “White isn’t a race” or “Black isn’t a race.” The reasons this is problematic should be obvious, but I’ll delve into them a little. Inasmuch as race is socially constructed, it is real. As long as racism exists in our culture and in our institutions, there may be no taxonomic validity to the idea of race, but it absolutely effects people’s lives. If that’s your argument for why you can make bigoted statements about one group (without being racist,) it applies equally to any group.

All that said, there is still a broader problem. Your claim is essentially that the person making the accusation has made a category error, one they could easily correct if they know the name for kind of bigotry you are engaged in.

“I’m not a racist, I’m a xenophobe!”
“I’m not a racist, I’m a religious bigot!”
“I’m not a racist, I’m an antisemite!”

Bigotry is bigotry. It doesn’t matter whether you call it a race, a religion, an ethnicity, a nationality, a gender, a sexual orientation, whatever. In any case, you are judging an arbitrary group of people based on an irrelevant characteristic. This is, then, essentially the same defense that J. Jonah Jameson called to his aid when accused of slander: “I resent that. Slander is spoken. In print, it’s libel.”

Either way it’s defamation.