You might look at some statistics about countries that have the largest Catholic populations and the declining or increasing number of Catholics in the population. Here’s one study:
pewforum.org/2013/02/13/the-global-catholic-population/
Then you might research the number of atheists in the world and what countries they are in and how those trends have changed over the years. Also, you’ll probably have to make some decisions on whether you want to include all Catholics and all Protestants in your research, or just practicing Catholics and practicing Protestants.
Also, how would you determine what the criteria for keeping faith in God is? Is it just being baptized Catholic, or would you want to do studies on abortion rates, mass attendance, pre- and extra-marital sex, divorce rates, promotion/acceptance of same sex marriage, birth control, belief in the precepts of the Catholic church, etc.
Here’s a Wikipedia article on the demographics of atheism that might help steer you in the right direction. I was surprised to read the results of the Canadian poll that found so many weekly church attenders don’t believe in God.
en.wikipedia.org/wiki/Demographics_of_atheism
My instincts say that being European leads to being atheist more than anything. And many European countries are traditionally Catholic.