Wrong, this has nothing to do with nationalism (source:- me; born and raised as Hindu, now a Catholic convert)

This has to do with Hindu cultural identity. The core of Hindu cultural identity (not Hindu religious identity, which is far more nebulous) is the caste system. The caste system does not work without the ones at the bottom buying into and believing that their status in this world is indeed wretched and deservedly so because of past karma.

Christianity, and any other proselytizing religion for that matter, is therefore attractive to those at the bottom rungs of society, who are suffering it's consequences, not the ones at the top who reap the benefits of privilege-of-birth. If the people at the bottom convert and leave for other religions, and thus find new identity, self-belief and a place under the sun, then it threatens the very edifice Hinduism in Indian society stands upon.

Now many people say casteism is a thing of the past and would like to brush this unappealing thing under the carpet and show you the shiny new buildings instead. But the fact is that caste remains the de facto fault line in Hindu society. Everything is determined by caste - what people eat, whom they marry, whom they do business with, what professions they have a stronghold on, etc.

Muslim rulers in the past were able to get away with conversions because those were indeed forcible (not that many in the lower rungs needed much incentive), or at least appeared forcible to the majority Hindu populace under the yoke of foreign rulers. On top of it, Islam set the tone for what proselytizing religions look like to Hindus, notwithstanding Christianity's differing impetus on salvation through Christ and his message of peace and love as opposed to Islam's dire warnings about hell and suffering for the Kafirs. So Hindu India has always been very leery of any conversion activity.

The reason Mother Teresa and a lot of other Christian organizations got to have a significant presence was because they weren't into converting people as much as providing education, relief and care. (although she was always accused of it)

Now that a Hindu nationalist party is ruling, grassroots Hindu organizations feel a lot more emboldened to try and reverse some of the years of attrition they had so to speak in the hinterlands. Mind you, nobody would have normally cared about these people - poor and rural as they are - but for the fact that missionaries work with precisely these kinds of downtrodden, left behind people.

Even as someone who left behind Hinduism, I do feel that Hinduism is not all that bad, other than the caste system. Hinduism is more an umbrella for similar philosophical and religious thoughts that sprung up at different times in history with the earliest going back to the Rig Vedic period - about 5000 years ago, and the very first line for the Rig Veda is "let good things come to us from all sides" - showing an openness to spirituality that is not restrictive. But time and again Hinduism has had revivals where it usurped challengers like Buddhism and Jainism and made them quasi-hindu philosophies and the more fundamentalist ideas have taken hold (sometimes in response to invaders and opposing religions such as islam)