Why did the Portuguese mistakenly expect to find Christians in India?

The Portuguese expectation of finding Christians in India was rooted in the legend of Prester John, a mythical Christian monarch believed to rule a powerful kingdom in the East, cut off from the rest of Christendom by Muslim expansion.123

Portuguese explorers, including Vasco da Gama, hoped that by reaching India, they would locate this legendary ruler and his people. They believed that establishing contact with these supposed Christians would allow them to form an alliance to strike at Muslim power from the rear and secure control over the lucrative spice trade.13

Upon arriving in India, the Portuguese mistakenly identified the local Hindu population as Christians, a misunderstanding that persisted until they became more familiar with the region's actual religious landscape. While they did eventually encounter indigenous Thomas Christians in Kerala, the initial, broader expectation of a vast, powerful Christian empire in the East was based on the enduring influence of the Prester John legend.314