There is still a surviving population of Thomasian Christians in south India today. Christianity obviously is going to appeal to the downtrodden and oppressed castes. It's a shame it didn't spread more than it did in those days.
Not just a surviving population- we're a reasonable percentage of the population of Kerala.
Christianity interestingly
didn't seem to appeal to the downtrodden and oppressed castes at the time. St Thomas Christians (known as Syrian Christians or
Suriani in Kerala) were happily integrated into the local caste system (there's some evidence, from Syrian Christian church vocabulary, that a number of conversions came from Buddhists who were eager to find a way to preserve status in the face of resurgent Hindu states). By the second millennium AD, St Thomas Christians actually often served as an intermediary class between Hindu aristocrats and their Hindu serfs- since accepting taxes from the lower caste peasants would have been ritually polluting to the upper castes, they contracted out the taxes to Suriani tax farmers who, being regarded as upper caste in status could then pay the revenue to the aristocrats.
When the Portuguese came,
they specifically did a lot of work among the lower castes which leads to an interesting social situation even in modern Kerala where the many groups of Syrian Christians are generally regarded as being of higher caste status than the Roman Catholics, and even Syrian Christians who accept Papal supremacy are careful to describe themselves as Syrian Catholic not Roman Catholic.