In the Kabul of the 1980s, my teacher asked an apparently harmless question: "Which one of you knows how to pray and can demonstrate it to the class?" I raised my hand before anyone else and was soon marching towards the front of the classroom. I put my headscarf in order and made sure my arms were hanging down straight on either side of my body and that my hands were open. This gesture, arms down and hands open, was the first thing I had been taught about praying. There was emphasis during the instruction that the arms be down and the hands open so that it is clear that you have nothing to hide from God.

I remembered this point of caution and followed it. But just then, when I started to say "in the name of Allah", there was turmoil in the classroom. Horrified shrieks turned the silent afternoon into a hysterical cacophony. "What you are doing is wrong!", the girls screeched. Some of them were standing up in protest. I shot back, "How can it be wrong when this is how my grandma prays?"

I had appealed to the taboo that no young girl ever had the right to contradict an old woman. But the taboo failed and if anything, the screaming became louder. This time it was about my arms. "Quick, hold your arms folded over your chest!"

My teacher used a typically Afghan method of problem solving. Without a word, she sent me back to my seat and turned to an eager shouter. "Are you a Sunni?" The girl nodded. "Hanafi?" Again, there was a nod. The teacher took a breath and started the act anew. This time around, everyone was happy – the girl held her arms folded over her chest. The theological disaster, innocently personified by me, soon disappeared as if by magic and order was restored in the universe. In Afghan Sufi tales, such scenarios typically end with: "And the believers shed tears of joy for a catastrophe was prevented and no one was harmed in the process."

I had not intended to traumatise my Sunni classmates. After all, the mystery of folded versus straight arms had never been explained to me. Nothing was explained in Afghan society. Everything was a given. We were all-knowing in our ignorance.

Later I learned that the folded arms had to do with Shia paranoia about Sunnis. Hardcore Shia assumed that Sunnis held their arms over their chests to secretly pray to an imaginary idol. In other words, in such people's suspicion, Sunnis were assumed to be crypto-idol worshippers, carrying on with the old, pre-Islamic faith while pretending to be Muslims. I realised that back in Kabul we had all been victims of an inherited inter-Muslim historical conflict, complete with the terror of the straight arms and the hallucination of imaginary idols.

The Shia faith that I knew was contemplative and melancholic, with a taste for philosophy, classical poetry and silent suffering. It somewhat resembled Catholicism but without the Catholics' glitter, gold and candles. Black, the colour of mourning, was the favourite shade. Only the Messiah could lift the mood and he, alas, was yet to come.

By contrast to this passive suffering, the active militant Shia notorious today was historically new and directly linked to Ayatollah Khomeini's politics. Like all other regional transformations, this change, too, spilled over the borders into Afghanistan, via immigrants exposed to political Shiism in Iran.

In 2001, when the Taliban's monopoly over faith was lifted, Afghan Shia returned, armed with a new self-confidence gained in Qom, Tehran and Mashhad. They started to "out" themselves, doing public mourning marches and adding the stamp of their distinct identity on the diverse mosaic of the post-Taliban society.

But their temerity ended in tragedy in 2011, with the first violent sectarian attack in Kabul. The problem that my teacher had made disappear as if by magic had returned full force, complete with 55 deaths in a suicide attack. Young Afghan photographer Massoud Hossaini captured its essence in a shot of a screaming girl surrounded by corpses. It was Karbala all over again, only this time in Kabul itself.

Hossaini won the Pulitzer prize for news photography and was quoted as saying: "I was born in a wrong place, Afghanistan, grew up in a wrong place, Iran, [and am] living in a wrong place, Kabul." His sense of alienation resonated with me. After all, all our wars had their origins elsewhere, in faraway places and long ago pasts. We were trapped in the Muslim history of unresolved conflicts – a host of superstitions and paranoia that was handed down to us as if they were our own. If I got away with it, with humiliation in front of my classroom, those who came after me paid with their lives.