Listen to this kirtan performed by Harjinder Singh and his Jetha - Hey Gobind Hey Gopal
The harmonium is the only instrument most Sikhs have ever experienced Sikh Kirtan through. It came into India via Portuguese missionaries as a more portable alternative to the church organ. The contribution the harmonium has made to Kirtan is huge. This is because it enables a Kirtani to learn to perform a Shabad within two weeks or so.
As the harmonium became more popular traditional stringed instruments started to go extinct, which is a fascinating thing.
I attended a Divan where 17 Namdharis came. They sang Kirtan without the harmonium. There were 3 joris, 4 dilrubas, a flute, and tarshanai. It was mind blowing. I decided I needed to find a teacher to learn a stringed instrument. It was difficult to find a teacher but eventually I found Surjit Singh (Namdhari). People were not impressed with me because the Namdharis had been vilified. They have been underrated in their contribution because they gave the Panth the gift of preserving the traditional stringed instruments. Every Sikh who plays a stringed instrument (at least in the UK) learned from the Namdharis somewhere down the line.
Even as a player and teacher of stringed instruments, I don't hate the harmonium. It has become a 'Sikh instrument' because we've been using it for more than one hundred years. Its become part of the fabric of our culture and an installment in our Gurdwaras. Great musical heroes have adopted it like Dharam Singh Zakhmi, and Sant Sujan Singh. We can't suddenly cast out that instrument.