When the Portuguese finally rounded Africa and made it to India they were convinced they had met up with Christians in India. To them everyone was either a Christian or a Muslim and they completely misinterpreted what the Indians were saying about their religion. Therefor they were convinced that they could get the Indians to side with them in a war against the Muslims. It took many years before the realized that the Indians were not Christians.

(Book – The Last Crusade: The Epic Voyages of Vasco Da Gama – Nigel Cliff)