I am not sure if the Catholic church has started to lose followers lately, I think they have, but they sure have been successful as a church in the past as far as getting a crowd. In Brazil, I have heard that the Catholic population to be as high as 85-90% and that is just one country out of many that have Catholicism as their main religion. Why is that so?
I was thinking this morning about a conversation I had in Sunday school a while back with someone who was not in line with our Baptist belief. She was convinced that her friend, who was not religious, but very good, would not be sent to hell by God. "He, in fact, was better than most Christians she knew." And you know what, it makes sense. I think that is why Catholicism has been successful, because according to our natural beliefs, it makes sense. We all tend to know what is "fair" and always want to be treated fairly. Catholicism says, basically, that if we do more good than bad, we will go to heaven. And that is what comes naturally to believe. And since everyone believes themselves to be basically good in their own eyes, we will all go to heaven.
That is the main reason I think it has done well, but there are others like having an official representative for God, offering plenty of works based services, being state run often times, and other things that I can't think of right now. But for now, here is a video to watch: