Assuming that the early Christian writings, supported by archaeological evidence and old local traditions, are true with respect to accounts of the apostle Thomas in India, it would be interesting to compare the Brahmans of Kerala's acceptance of Christianity with the non-Brahman castes' acceptance of alternative Hinduism and Buddhism.
Why does this difference seem important? According to Dr. Tennent's lecture, India was ripe for Buddhism because the Kshatriyas were seeking ways to promote their own political power over the Brahmans' spiritual power. The shift away from the caste-justifying ("legalistic"?) Vedas and towards the philosophical ("touchy-feely"?) Upanishads was to benefit the warriors class greatly because the kings desired religious support for their claims against the priests' monopoly on power. Buddhism, with it's message of equality, would have appealed to the lower castes.
To summarize, these movements would have been attractive alternatives to strong Veddic Hinduism because they would unseat those in charge. In such cases, personal lust for power or a malicious desire to see someone else's downfall would become strong motivators for promoting religious change. A known case of this occurring was when Martin Luther received considerable support from German princes who, rather than being convinced by his theological arguments, were anxious to remove themselves from the Holy Roman Empire's authority and establish their own power.
How does this relate to Thomas's missions to the ancient Brahmans? Well, Christianity, like these two movements, had a message that attacked strong Veddic Hinduism's oppressive caste system. However, unlike these two movements, this took root among those in power, the priestly families. By converting to Christianity, the Brahmans jeopardized their own status, not that of others. Even those factions that ended up retaining much of the caste system would have been aware of the social implications of Christian teachings on eating and conversing with the lowest in society. This would have been an especially dangerous position to take because of the fear of becoming "outcasted."
In conclusion, reforms in Hinduism and the development of Buddhism could be interpreted as being extremely self-seeking on the part the lower castes' members. In contrast, the initial adoption of Christianity by the Brahman could be seen as self-sacrificing, a true witness to the world. This is not to say that Christianity cannot be a message of comfort and hope to the oppressed, but it is to say that the conversion of the oppressor has very different implications.