⟩ What would you do if the seat belt signs were on and the aircraft is about to take off, but a passenger insist that he has to kneel down to conduct his prayer due to religious reasons?
Well obviously this is a delicate situation. However first and foremost is Passenger safety.
I would deal with this situation by calmly speaking to the passenger and telling him we respect his choice to pray but he cannot do this during take-off because of the danger to himself and other passengers. I would politely ask him to take his seat.
Also if I were a junior Cabin Crew I would confer with a senior as there are many things involved.