There is indeed a very old Christian community in Kerala that plausibly dates back to a St Thomas or something like him. The community itself considers St Thomas to have been its founding missionary, and while this is unverifiable because there are no records that date to the first century and is thus something of a foundation myth, it is at least an extremely old foundation myth. Christian writers as early as the third and fourth century discuss the community as being founded by St Thomas or St Bartholomew--Eusebius describes the missionary work of his teacher, who traveled to India and found a well established Christian community already present.

There is also a historical context, in that the middle first century (which is when St Thomas is said to have done his work) was the height of the trade going between Egypt's Red Sea coast and southern India. What this adds up to is that the traditional account is plausible, and even if it was not literally founded by an apostle in 50 CE, that account is not terribly far off the mark.

Grant Parker has a useful discussion of this in The Making of Roman India.

Straight away we can say that India constitutes the seventh largest Christian population on Earth, with over seventy million followers. In areas of particular association with Thomas the community can be as much as 7-8% Christian. How much of this is down to Thomas?

Today, no serious scholar outside of India would endorse a literal interpretation of the Thomas tradition. We have no evidence that the Apostle Thomas was an active missionary in India and several parts of his traditional veneration in that country are patently nonsensical. Lets take a look at a couple:

Atop Chennai's St. Thomas Mount is housed the cross on which the Apostle collapsed during his death. Such a cross would not develop into a spiritual signifier until the fourth century, which makes this tale suspicious.

Similarly, Thomas is associated with a painting of Mary which was made by the Apostle Luke. This is not an unusual subject and Chennai holds one such example. However, the cult of Mary did not appear until the second century, which makes it unlikely that a representation of her would be conveyed over a continent into India. These traditions suggest that the presence of Thomas has been retrospectively constructed by later believers.

The situation is further complicated by the fact that the Acts of Thomas was formally discounted by the Catholic Church in the 16th century. The other sources for Thomas' mission in India (and Indian Christianity in general) are likewise from the fourth century.

None of this should dissuade you from interest in the Thomas Christian church, because it is fascinating. Practically no other Christian community developed in such an insular, idiosyncratic fashion and it predates many European Churches by the best part of a millennium.