There's no "evidence" the Catholic justices are "more liberal". If anything, the opposite is true. The MOST conservative justices in the court in recent years were Catholics: Scalia, Alito & Thomas. Most of the liberal justices in recent years have been Jews and protestants, like Ruth Bader Ginsburg and John Paul Stevens (which doesn't "prove" protestant judges as a whole tend to be liberal, but was simply a coincidence).
In fact, the only "liberal" Catholic justice in recent years was Sonia Sotomayor, but by her own admission she's only "Catholic" on paper and doesn't practice the faith or go to church, which makes her as "Catholic" as Bernie Sanders is "Jewish" and Jesse Ventura is "Protestant".