The easiest way to do it would simply be having the Christian world out-breed everywhere else, say with statistics like European countries all having populations of more than 70 million, and perhaps each of the America's near or more than a billion.
It would be easier statistics wise if you'd asked how can we get 70% to follow an Abrahamic religion (51% of the world do OTL) rather than making it that high in terms of Christianity (just over 31% OTL follow Christianity.), but yeah, easiest way is to just have them breed alot.