India has been exposed to Christianity for ages before the missionaries...St Thomas claim whether true or not likewise the traders have been exposed to the religion..
The conversions inSouthTamilnadu was again during the portuguese and dutch peroid where people converted for various reasons...
The west Coast particularly Kerala the conversions were because of strong caste based divisions in the kerala hindu community which led to people embrasing other faiths and not lack of Hindu Monarchs
As far asthanjavut if you go to papanasam and rajagiri areas you will be surprised by thenumber of muslim families in that area..
cristianity has always had strong roots in thanjavur the diacese education centre where the st Antonys and Sacred Heart school are supposedly present from early 1800s
St Peters School is supposedly the first school to teach St. Peter’s school established by Rev. C. F. Schwartz in 1784. This is the first school in India in which English was taught to Indians. The school was substantially supported by Raja Serfojee. It has been located in or near the fort. Many great educationists like Dr. J. Marsh, Rev. W.H.Blake, Rev. A.H. Lamb, Rev. G. U. Pope contributed to the performance of the school.