Indian Christians believe that Christianity was brought there by Thomas and the church there goes back to the first century.
Supposedly when the first Portuguese explorers arrived they found native churches already in place.
There’s no “supposedly” about it, the Portuguese found indigenous Christian churches in India. Parts of India were Christianized by the Apostle Thomas (according to the uniform tradition of all the Eastern Churches, Orthodox, monophysite or Nestorian). During its early history the Indian church was under the (Nestorian) Church of the East, centered in what is now Iraq. After the rise of Islam cut them off from their mother church, they eventually reestablished contact with Christians in the Levant, somewhat bizarrely completely switching Christology in the process and are now in full communion with the monophysites (Copts, Ethiopians, Syrian Jacobites, and Armenians), well other than the ones who, under Portuguese blandishments, became Uniates, keeping their native liturgies and entering communion with Rome.