I’m not sure how much familiarity you have with Eastern Catholicism but Knanaya Catholics are an ancient community of Catholics who are under the Syro Malabar Catholic Church which is one of the twenty-two Eastern Catholic Churches. Long story short, Knanayas have had a tradition of endogamy in order to preserve their small communities ancient Christian Customs amongst the larger community, it is also a way to preserve their identity. In the early 20th century when the Syro Malabar Church was restored in status and diocese were erected, one was created exclusively for Knanaya Catholics which is now today’s Kottayam Archdiocese of Kerala India. For the longest time Kottayam Archdiocese has acted as a sort of Sub-Sui Juris (particular church) with the customs and traditions of Knanaya Catholics.
Yes it is true that Kottayam Archdiocese does not accept converts because of the endgoamous tradition but Knanaya Catholics do highly evangelize, tho all converts are transferred to the near by Syro Malabar Diocese, once again because of the exclusivity of Kottayam Archdiocese (you must be born by two Knanaya parents in order to be considered a member). When it comes time for marriage if you break tradition and do not join Holy Matrimony with another Knanaya you loose membership to Kottayam Archdiocese and either join the Church of your spouse or another diocese of the Syro Malabar Church.
Tho many Christian Communities maintain a slight level of endogamy, none else practice it as strictly as the Knanaya Christians. It is the only group within the Catholic Church with such a practice and an ancient set of traditions. To learn about Knanaya Catholics and Kottayam Archdiocese please visit -
kottayamad.org/