"Well, you’re not really that Jewish, are you? Your dad’s half-Jewish and your mum was raised a Christian. So actually you’re only a quarter Jewish. In fact, I’d say you had Jewish heritage. I wouldn’t actually describe you as Jewish at all.”

The words stung my friend’s daughter. She knew the boy was trying to hurt her, that it was payback for having declined his romantic advances the other evening, but hot tears still welled up in Sophie’s eyes. Blinking furiously, she looked round wildly at the others, willing them to rebuff him. But the Jewish teens giggled nervously, shuffled a bit and said nothing.

When my friend told me about the incident I thought back to my younger years in Cardiff and my dealings with a girl called Alison. I must have been about seven when my parents told me that there was one other Jewish child in my class at Rhydypenau Primary School, and I clearly remember the warm feeling of kinship that swelled in me at the discovery, the pleasure of not feeling like an island.

Unfortunately for my younger self, Alison saw things differently. When I excitedly announced our shared ethnicity to her in the playground she scowled, and muttered something about her parents saying my family wasn’t properly Jewish.

Not properly Jewish? I didn’t understand what she meant. In my parents’ home, there were no degrees of Jewishness: you were either Jewish, or you were not. I did, however, understand I was being rejected, and that hurt. So I resolved to show Alison she was wrong and went to school the next day armed with the Star of David chain my refugee paternal grandfather, Hirsch Baruch Glaser, who left Czechoslovakia in 1938, had given me for my seventh birthday. I felt sure it was the proof I needed.

Back then, though, my complicated Jewish background — in brief, both my parents discovered they were Jewish in their late teens —meant my Yiddishe vocabularly was not as advanced as Alison’s. So when I produced my Jewish necklace, as I called it, for her approval, I effectively hoist myself by my own pendant. “You see!” declared my seven-year-old detractor. “You don’t even know it’s called a Star of David. If you were Jewish, you would!”

Rejection hurts wherever it is delivered, but it feels particularly poignant that Sophie’s encounter with Jewish cliquiness was when she was on tour in Israel where people with one Jewish grandparent have had the right to live since 1970, whether or not they are considered Jewish under Orthodox interpretations of halachah. Put another way, Sophie is kosher enough for the most Jewish place on earth, but not quite Jewish enough, it seems, for British teens on Israel tour.

What on earth is the reason for this cliquiness that strays, so easily, into ostracism, for this nasty notion of illusory purity? Why does it feel good to say I am more Jewish than you? A rabbi friend of mine describes it as the community’s least appealing trait, and she’s right. But I don’t think she and her fellow Progressive minsters are the source of the problem: Reform and Liberal rabbis and communal leaders all speak the language of inclusion we want to hear.

I think the problem starts in the home. “Josh and Zack have known each other since they were three!” Or: “Me and Hannah were pregnant at the same time, so our kids were always going to grow up together.” We have all heard variations on these statements, and we all know they are made with pride: long standing friendships are a big, shiny badge of honour in our community. And what’s wrong with that, you might ask. Nothing unless it encourages, as I believe it does, our kids to close ranks.

The parental anxiety that our kids don’t have enough Jewish friends — which we have all also heard — springs from the same mindset. It promotes, albeit often subliminally, the concept of exclusivity rather inclusivity.

Psychologically, I understand why Jews feel the need to stick together: antisemitism. But by pushing out Jews who have one Jewish grandparent rather than four, or by being suspicious of Jews who have never set foot in a synagogue, or by not wanting your son to marry a convert, we aren’t defending ourselves against the enemy, we are closing the door on friends — Jewish friends.

I said all this to my friend’s daughter, and I added something else. “Never let other people define who you are, Sophie. Particularly when they are being racist.”