Although that sounds like a Freakonomics type explanation, it seems pretty unlikely that smoking and drinking rates would not already have been assessed and excluded. These are two pretty big factors at the top of the list when examining life expectancy.

Also, if your hypothesis was true, then it would need to hold for other [Roman] Catholic groups for that to be the key factor. That does not seem to be the case as, according to Wolfram Alpha, the top life expectancy (at 84.4 years) is Macau, followed by Japan (83.6) then Switzerland (82.6) and Australia (82.5) tied with Andorra. Italy is 6th though.

While there are Roman Catholic countries high up, there are also higher ranked, non-Roman Catholic ones.

There are many convincing cases of benefit derived from religious faith but I'm not sure you can make such a case for life expectancy.