Archive.org direct link
Open Library main page
Edavalikel Philipos - The Syrian Christians of Malabar: otherwise called the Christians of S. Thomas (1869)
Archive.org direct link
Open Library main page
Richard Collins - Missionary Enterprise in the East, with especial reference to the Syrian Christians of Malabar (1873)
Archive.org direct link
Open Library main page
Michael Geddes - The History of the Church of Malabar (1694)
Archive.org direct link
Open Library main page
W.J. Richards - The Indian Christians of St. Thomas : otherwise called the Syrian Christians of Malabar (1908)
Hathi Trust link
The way I heard the story is that late in the 15th century Vasco de Gama made his first major voyage, intending to open new trade routes to India. In 1498 he landed on the southwestern tip of India (modern Kerala) and was surprised to discover that there was already a thriving community of Christians there, moreover ones practicing older beliefs. In my mind I somewhat imagined this like the scene in Monty Python and the Holy Grail where the knights reach a castle, ask if the inhabitants want to join the quest for the Grail, and are dumbfounded to receive the answer that the inhabitants already have one.
This group is called St Thomas Christians after the story that the apostle Thomas visited in 52 CE and established their church. (They're also called Syrian Christians after early Syriac speakers who immigrated and settled there.) Though the story about Thomas seems likely to not be true there's just enough plausible evidence that it can't be dismissed out of hand. There are even claims that his tomb is nearby - you can visit it (or at least one possibility). Vasco's landing prompted the arrival of priests a couple of years later which threw the St Thomas Christians, who were fully organized and well functioning, into turmoil that seems to have never entirely settled. You can read a recap at the Brittanica or a detailed one with more history and theology at the Catholic Encyclopedia.
In any case, the story I heard was only part of it. The St Thomas Christians weren't completely unknown in Europe since there had been European visitors in late antiquity and the middle ages. Marco Polo even made a short report about them and there were Church travellers in the 14th century. In fact I have a suspicion that the Vasco story may not be completely accurate because I've only seen it repeated in second-hand sources but don't have time to check biographies or the voyage journals. There should be much more detail in N.M. Mathew's St. Thomas Christians of Malabar through Ages: A Fresh Look into Biblical and Historical Evidences (2003), Benedict Vadakkekara's Origin of India's St Thomas Christians: A Historiographical Critique (1995), Peter C. Phan's Christianities in Asia (2011), Em nome de Deus: The Journal of the First Voyage of Vasco da Gama to India, 1497-1499 (2009, edited by Glenn J. Ames) and Nigel Cliff's The Last Crusade: The Epic Voyages of Vasco de Gama (2011).
