I chose this article from Pew Research, "Most Americans don’t see Democratic candidates as very religious", as it's timely and gives a number of avenues for further investigation--especially as the table with the drill-down numbers indicates some group/candidate pairings may vary quieta bit from the overall numbers.
Pew Research can be a good place to start if you need background data, or to see how much of the research leading to an investigation has already been performed. Pew also provides datasets for analysis.