Maybe this--no Islam, but we have some major Yemeni state convert to Nestorianism and remain powerful for a few centuries. Their trade with Kerala would reinforce the St. Thomas Christians, and eventually, at least one Keralan ruler would convert (you might be able to have this with a spontaneous conversion of a Kerelan ruler, but I'm not too sure of the plausibility). Even if this Yemeni state collapses and another religion takes over, hopefully Christianity is still established in India. Other trading areas in Tamil Nadu and Gujarat would hopefully also be host to a large Christian minority, and some smaller states would be totally Christian with Christian rulers even if they're nominally ruled by Hindus or whatever. The end result means we'll have at least a couple of Indian social groups which are entirely Christian, some maybe well-scattered throughout India. In time, a powerful regional Christian state might emerge and be able to convert a significant amount of its people to Christianity, even if it will have to have some manner of religious tolerance.
So we can probably have at least a large part of Kerala following Christianity, along with scattered communities elsewhere in India. Europeans will no doubt be interested by these large Christian communities, but probably disdainful of them due to their heretical nature. If we somehow assume a "united India" (without colonialism, this is obviously very difficult, but either an insanely brilliant conquerer and his empire survives and unites the subcontinent or Indian states merge into a federalised "Indian Union"), then Christians would make up a sizable minority in this Indian state.