In the late 15th Century, when Catholic missionaries from Portugal arrived in India to convert the "heathen", they found a well-established Christian community already there. Local tradition had it that this had been founded by St Thomas the Apostle, possibly as early as 52 AD.
Also, that term "Orthodox" is a rather general term which covers quite a spectrum of different churches, including the likes of the Egyptian Coptic Church and the Ethiopian Coptic Church. Established in Alexandria - and claiming the year in 33 AD as its founding date - the Egyptian Coptic Church is certainly one of the oldest Christian churches in the world. With such a diverse mix of churches under the general classification of "Orthodox", it almost goes without saying that they don't always agree with one another! Which shades further doubt on the idea that there ever was such a thing as a "Pure Church".