Well, i do not think anyone knows of any answer except guess work. Whenever it was, it is unlikely that missionaries introduced Christianity to India. on the believer front, i think they are on the increase as many new missionaries are finding it quite easy to infiltrate in to poverty stricken people and buy-out their faith.
So little is know about the life of Jesus Christ before the advent of His ministries that speculations as to His travels abound. However, their is evidence that the Apostle Thomas of the New Testament and Acts of the Apostles did, indeed, enter India and founded a the Christian Church in Madras India during some period in the first century. This is a fact and recorded in the works of Origin the Christan historian who lived in Alexandria Egypt in the second century of our era. Accordingly the Christan Church found its' roots in the first century shortly after the death of Jesus Christ.
It may be of some value to investigate the similarities between Thomas and Jude the Apostles'. It is strongly suggested that these were both one and the same individual and a kinship by blood may have existed between Thomas or Jude and Mary the Mother of Jesus. For your benefit I have posted two links below.
when did christianity entered india and how many people were following it since the e
