You're overlooking the Portuguese who were there before, during, and after the British colonial period. They not only established settlements and converted non-Christians, but also converted a large portion of Keralan Christians to Catholicism. Protestant missionary activity started two centuries later, with large concentrations found primarily in tribal areas and the northeast.

It has already been mentioned but the Portugese had an enormous influence in this. Vasco de Gama landed in Goa hundreds of years before the British took power. De Gama first landed in 1499, bringing with him priests and missionaries as was custom in 15th century Portuguese exploration. He slowly built up a fully fledged colony that took over large part of India. And, as with all Iberian colonies, the Portuguese inquisition was introduced. This leads on to one of my favourite facts: the Portuguese inquisition didn't officially end until go a was returned to India in 1961. The inquisition forced mass conversion and is responsible for a lot of the Catholics.

On the opposite side of the subcontinent, Pondichicherry was owned by the French for many years. Less oppressive but definitely very Catholic.

And on to the British, who ruled for nearly a century from c.1850. The British at this time were significantly less interested in converting their subjects than the Portuguese had been. They were more driven by wealth and a sense of proper Victorian society. The British were also relatively content to leave the various religious groups alone. A prominent historical viewpoint is that the British were very good at ruling through 'Divide and Conquer' governance. This, plus a healthy dose of racism - Jesus was for good Victorians and not for the n*ggers (a term used widely throughout the British raj for all non white people).