About a hundred years ago, Hillaire Belloc famously said, Europe is the Church and the Church is Europe. It was Europe who sent missionaries bearing the light of faith to all lands. I think we're seeing a changing dynamic, where perhaps the day is not too far off, when it will be those from Asia and Africa who will bring that light of Christ back to lands where it has been snuffed out, or almost lost.
Telespohorus, I looked up what you asked, and the approximate population figures are 100 million, 500 million and 1 bilion for 1900, 1978 and today. Or there were 53 million new Catholics out of an increase of 400 million in the population and 110 million out of a further increase of 500 million. So that is still a growth in the Catholic percentage of the total population, as such, from 2% to 11% to 16.5%, which is not bad.
But the increased vocations to the priesthood and religious life are waht I find heartening. I don't deny that the leaders of the Church, on the human side, share some of the blame. But I think the disparity in vocations in this timeframe between say, Africa and North America comparatively, make it likely that at least a large part is cultural rather than universal to the Church.
Rhea, well the Protestants have grown too, but it's scattered as you'd expect across their various denominations.
Graham, no doubt some left because Church leaders simply didn't seem to reflect Catholic beliefs anymore. But I think a fair number also left because the Church wasn't hip enough. As early as 1968, there was a widespread dissenting response to Humanae Vitae, and obviously things were going to get much worse as people actually began to engage in sin and to approve it culturally. Also, recent times have increased occasions for sin of all types.
Francisco, I agree with you, even though I think there has been some Vatican interest in Africa in recent times. The Pope recently went there and mentioned one of his Cardinal friends he was close to, I forgot his name. I've also heard though that some seminaries are even more liberal than the general population.