The growth is a internal development with Chinese doing most of the conversions. The Communist effort to expel the foreign missionaries actually increased Christianity's successes because the cultural gap between missionaries and subjects was eliminated.
Protestants and Catholics are playing two different games when it comes to converting. The Catholic Church is now trying to negotiate and bring the "official" church in China back into communion. Pentecostal Protestants however are bypassing government controls and setting up their own house churches. So far the Protestants are winning.
Christianity is old in China but ever since the Nestorians were outlawed (it is rumored they nearly converted an emperor). Today the emptiness of a post-communist (not post-Communist) culture has Chinese searching for answers in religion. Christianity and Islam are leading the charge in gaining new followers.
So what does this all mean? Well, in the future the Pope has a growing chance at being Chinese (think what John Paul II was to the Soviets), an African-Chinese tag team might knock on your door asking if you know about Jesus, and of course there is also geopolitics. Would a more Christian China be open to better relations with the West? Time may tell.