I don’t think Italy’s decline can be attributed to one party. Unlike France and Italy, who have been nation-states for centuries, Italy has been united for less than 200 years. I think, generally speaking, it’s various ideologies of the left that have brought it down, just like the rest of Europe.
I wonder if Europe hasn’t been choked with unbelievers for 1500 years. Only difference now is people feel free to admit their unbelief.