[This post is my latest comment in response to Bruce Charlton's post on his blog Bruce Charlton's Miscellany, titled “Discerning truth and goodness from experience of fruits: the example of Mormonism”. I'm posting my comment here since it's too long to fit in his combox.]
Is the question, whether the belief in and practice of each religion makes a difference to people who believe in and practice it; or is it, how effective is each religion in getting its members to practice it actively?
In answer to the first question, I do not concede that people who believe in and devoutly practice the Catholic religion derive less benefit from doing so; and I deny that the statistics you give prove the contrary.
In terms of the second question, I concede that the LDS Church is probably more effective at keeping its members active. And I think the reasons for its effectiveness are those that I gave: That it takes proactive steps to perpetuate a “Mormon culture” among its members, not least among children growing up in the Church; and in a studied manner takes steps to keep members active, to keep tabs on how active those members remain, and to follow-up on those who become inactive and make efforts at getting them to return. And not less importantly, monitors members’ compliance and makes them jump through hoops in order to take part in the most important aspects of LDS religious worship and practice.
I have conceded that the Catholic Church could learn some things in this regard. Other than “requiring” people to attend Mass under threat of sin (but with no method of enforcement); and offering religious instruction for the purpose of preparing children for the sacraments; there is really no longer any distinctive “Catholic culture”, as in fact there was prior to the time of Vatican II. This is a weakness in my view, but something that has always been a strong suit for Mormons. Why is it that Mormons have always been so careful to inculcate a specifically Mormon culture among its members, and remain so engaged in their lives? It’s an interesting question.
You say that among Catholics there is no measurable difference in their behavior compared with the mainstream population. But how are you defining “Catholics”? There are millions upon millions who, if you asked them, would say they were Catholic, by virtue of their family background or having been baptized as infants, but who are not “adherents” to the Catholic religion in any meaningful sense.
Thus in my view it’s not fair to ask whether the Catholic religion affects the behavior of Catholics to the same degree that the Mormon religion affects the behavior of Mormons. There are too many differences between the respective sets of “Catholics” and “Mormons”, in terms of culture, history, geographic distribution and sheer numbers. Rather the question should be, does genuine belief in and practice of each religion affect the behavior of its adherents.
You say there is no difference in behavior between students of Catholic colleges and those of mainstream colleges, whereas big difference exist between students of BYU compared with students of mainstream colleges.
But how are you defining a “Catholic college”? Is a typical American Catholic college comparable with BYU, in terms of them each being representative of their respective churches?
I would argue, no. BYU is owned and operated by the LDS Church and therefore under its direct control, whereas no American Catholic university is directly owned and run by the Catholic Church. Some are owned and operated by religious orders, but that’s not the same thing. In other words, the Pope has no authority over the activities and policies of American Catholic universities since they are independently owned and operated; unlike the few LDS Church-owned colleges.
There happen to be some Catholic colleges that I consider comparable to BYU in terms of the strict moral and religious standards that they require of faculty and students. There should be a helluva lot more, I admit. But the point is that when a Catholic college imposes such requirements on its students and faculty, and students choose to go there (the vast majority of Mormon college students don’t attend BYU), there is in fact a marked difference between the moral and religious environment of such a school compared with mainstream colleges. (I know because my son attends one such.)
Brigham Young evidently wrote of the prospective founding of BYU: “I hope to see an Academy established in Provo… at which the children of the Latter-day Saints can receive a good education unmixed with the pernicious atheistic influences that are found in so many of the higher schools of the country.”  The website of Christendom College says of its own origins, “Founded in 1977 in response to the devastating blow inflicted on Catholic higher education by the cultural revolution which swept across America in the 1960s, Christendom’s goal is to provide a truly Catholic education in fidelity to the Magisterium of the Catholic Church and thereby to prepare students for their role of restoring all things in Christ.”  Interesting parallel, indicating similar missions.
In other words, just as there are Catholics and there are “Catholics”, there are also Catholic colleges and “Catholic” colleges. It’s not fair to compare any number of “Catholic” colleges with one authentically Mormon one. That’s like comparing Mormons who practice their faith, with Catholics who don’t. I would stack the religious and moral environment of Christendom College or Thomas Aquinas College against that of BYU any day, including the depth and seriousness with which the students and faculty think about and practice their faith, and the effect that it has on their lives.
But more to the point, even if a lower percentage of those who call themselves Catholic, compared with those who call themselves Mormons, take their faith seriously and follow it conscientiously, I don’t believe it can be shown that those who do so derive less benefit from it, or behave less morally than their Mormon counterparts.
I have seen too many instances of the power of the Catholic faith to change lives, and too many examples of the good fruits that follow from genuine faith in its tenets, and devout participation in its sacraments. I have seen not merely good individual Catholics and good Catholic families, but entire communities of devout Catholic families, with several large families in the neighborhood of 8, 9, 10 and even 13 or 14 children, the vast majority of whom are happy, well-adjusted and decent. There is a clear difference between the behavior and character of Catholics who are lukewarm towards the faith, and those who truly believe and devoutly practice it, which clearly points up the ability of the faith to effect changes in behavior and character, when it is actually put into practice.