In the year 345 A.D., one Knai Thomman, a rich international merchant, and 72 families from Cana (Middle East), belonging to East Syrian Church (Chaldean), immigrated to Malabar (presently Kerala) and established their colony. In the group there were also priests, deacons and their bishop. Knai Thomman and his people were welcomed by Cheraman Perumal, the Emperor, and were given permission to settle down in Kodungalloor. They built a town in Kodungalloor with a church and 72 houses. The natives called it Mahadevar Pattanam meaning "town of superiors". Later, Cheraman Perumal bestowed them with many princely privileges, and it was recorded on copper plates (Knai Thomman Cheppedu) given to Knai Thomman.
Knanaya Christians are the descendants of these Jewish Christian immigrants in Kerala. They were also called Southists (Thekkumbhagar) because they lived on the south side of Kodungalloor. The St. Thomas Christians (native Christians of Kerala) who lived on the north side of Kodungalloor were known as Northists (Vadakkumbhagar). Another tradition is that Knananites settled down on the south side of Periyar river while the native Christians lived on the north side of the river. It is also stated that Knananites were called Southists because they came from the southern kingdom of Judah.
Knanaya Christians are seen not only among the Catholics but also among the Orthodox Christians of Kerala, because when a group of Thomas Christians joined Jacobitism, among them there were Knanaya Christians too. They observe many rituals and traditions which are very peculiar to their own community. Even today, the Knananites continue to be an endogamous community. In order to keep the purity of their race and ethnicity, they do not marry any one outside their community. They also keep away from evangelizing and converting others to Christianity. If anybody is converted to Christianity, such a person won't be accepted in the community. Though this sense of "purity" is a distorted one, and is against basic Christian mission, it is acknowledged by Rome by establishing a separate diocese (Kottayam) for the Knanaya Catholics. A Knanaya Catholic can marry a Knanaya Jacobite, but cannot marry another Catholic even one who belongs to the same rite! It seems, Knananites are more proud of being Jewish than being "Christian."
For more details, see The Early Christians of India and The Syro-Malabar Church.