Thanks. Until now, I knew nothing about these other groups joined in the Christian faith.
NYer has a chart of the non-Roman Catholic Churches which is extremely interesting. The earliest Christian communities in India are very persuasively dated to the first century mission of the Apostle Thomas, who was martyred in India. They long predate Islam in India.
Christians are kind of concentrated in the coastal areas of the southern half of India. Kerala State, for example, is about 50% Christian, including more than one Catholic Rite, Eastern Orthodox, and (for lack of a better term) non-Catholic, non-Orthodox congregations, some very ancient.