Why would some foreigners see the spread of christianity as a treat to their culture?

Generally, they would see Christianity as a foreign religion, which could undermine whatever religion is already in place (e. g. Hinduism or Islam). It's like diluting the religious 'purity' of their culture, in a way. As religion is generally an important part of a culture, this Christianity may in turn may 'damage' the culture, in the eyes of the people who live there.