What was the main reason behind the Sepoy Rebellion of 1857?

The main reason behind the Sepoy Rebellion of 1857 is actually multifaceted. The direct cause was a change in the use of the weapons Britain provided to Indian sepoys, basically soldiers fighting for the British. The British began to implement a new kind of grease for their guns that was cheaper than the previous grease. However, this grease was a combination of pig fat and cow fat. In order to open these packets of grease, one had to use their teeth. Remember that India's major religions at the time were Hindu and Islam. The Hindu sepoys were appalled by the notion that they would use the sacred cow, and the Muslim sepoys refused to bite open the packets because pork is taboo to them. This whole incident combined with general resentment towards British rule of India, and the fighting began.