Originally, the "Thomas Christians" of India were grouped into a metropolitanate of the Assyrian Church of the East.
Today, there are approximately 8 million descendants of the "original" Thomas Christians surviving but in 5 independent and separate Churches. Some may have joined the Latin Church, which has about 12 million faithful:
Ancient Churches of the East:
(1) Assyrian Church of the East (remnant of the original Thomas Christians)
(2) Syrian Orthodox Church;
(3) Malankara Orthodox Syrian Church
(4) Syro-Malabar Catholic Church; and
(5) Syro-Malankara Catholic Chruch.
The largest among them are the Syro-Malabar Catholics with about 4 million adherents followed by the Malankara Orthodox Syrian Church with about 2.5 million faithful.
Within each of these 2 Churches, there is a distinct ethnic community known as the “Southists,” or “Knanaya.” According to tradition, their origins can be traced to a group of 72 Jewish Christian families who immigrated to India from Mesopotamia in the year 345 A.D. and now number about 300,000.
There are two other (relatively) small churches in Kerala that originated in the Malankara Orthodox community: both protestants (Anglican-leaning):
(6) The Mar Thoma Syrian Church of Malabar; and
(7) The Malabar Independent Syrian Church of Thozhiyoor.
Recapitulating, the numbers as of end of 2004 (as estimated by Fr. Roberson of CNEWA and by other religious demographers) are:
Indian Oriental Orthodox: 3,715,000
Thomas Christians still under ACE: 15,000
Syrian Orthodox Church: 1,200,000 (+500,000 in Syria)
Malankara Orthodox Syrian Church: 2,500,000
Indian Oriental Catholics: 4,405,000
Syro-Malabar Catholic Church: 4,000,000
Syro-Malankara Catholic Church: 405,000
Indian "Protestants": 710,000
Mar Thoma Syrian Church of Malabar: 700,000
Malabar Independent Syrian Church of Thozhiyoor: 10,000
Indian Latin Rite Catholics: 12,000,000
If one adds the 500,000 non-Indian (i.e., Syrian or Arabic) faithful of the Syrian Orthodox Church, the Oriental Orthodox in India would total about 4,215,000 and become as large as the Oriental Catholics at 4,415,000.
However, if one adds back the 2 Mar Thoma "Protestants" to the Malankara Orthodox, then the Oriental Orthodox in India would total 4,925,000 or ~5 million, now slightly larger than the Oriental Catholics.
The Orientals (Orthodox and Catholic), are concentrated in Southern India, in the State of Kerala, while faithful of the Latin Rite Church are dispersed throughout India.
A caveat: Catholics comprise merely 1.6% (Christians: 2%) of India's total population of 1+ billion while Hindus number between 800 to 900 million! (Did you know that there are more Muslims than Christians in India? There are approximately 135 million Indian Muslims!)
Let's pray for our Indian brothers and sisters for the difficult job ahead!