It's hard to blame the old Protestants for what they did, for they didn't know where this road led. They were pioneers, they were cutting a path in the religious wilderness. They feared that traditional Christianity was becoming unbelievable; that if they didn't modernize their religion by dropping certain old-fashioned doctrines, modern men and women would no longer be able to accept Christianity.

As it turned out, to modernize Christianity, at least if you carry this modernization process beyond a certain limited point, is to destroy it. Look at the liberal Protestant denominations today. All of them are shrinking rapidly in numbers. All of them have lost much of their once-great social influence.

But liberal Catholics don't have this excuse. They can't very well say, "We didn't know where our liberalism was taking the Church." For they have the precedent of liberal Protestantism in front of them. Their ignorance is vincible -- and culpable.

Liberal Christians, beginning with the Boston Unitarians of the late 1700s and early 1800s, always "improve" Christianity according to the same pattern. The pattern is this: You attempt to blend what seems to you to be the essentials of Christianity with the best in whatever happens to be the fashionable anti-Christianity of the day. This synthesis, partly Christian and partly anti-Christian, will, of course, be incoherent; but at the moment you're creating it, it looks pretty good.

In the generation after the American Revolution, the fashionable form of anti-Christianity was Deism. And so the Boston Unitarians said in effect, "While Deism is very wrong in its rejection of Christianity, the Deists, it must be admitted, make a few good points. So let's toss out the Trinity and the Divinity of Christ and Original Sin. We'll then have a purified Christianity."

In the post-Civil War era, the fashionable form of anti-Christianity was a triple-headed threat: (1) agnosticism; (2) evolutionary theory; plus (3) a skeptical higher criticism of the Bible. Liberal Protestants responded by becoming near-agnostics while arguing that Christianity is far more about morality than knowledge: doctrine is of little real importance.

They became evolutionists, holding not just that biological species have evolved (under God's guidance) but that religion itself has evolved, Christianity being but its latest result, and we should expect more evolution in the future. As for the Bible . . . oh, well. It abounds in errors, but it's still a very good book.

In the 1960s and 1970s the fashionable form of anti-Christianity was the sexual revolution -- a total rejection of traditional Christian sexual morality and, by implication, a rejection of virtually all the rest of Christian teaching; for if Christianity had been wrong for all these centuries about sex, wasn't it likely that it was wrong about almost everything else?

As usual, liberal Protestants responded by blending Christianity (what was left of it) with this form of anti-Christianity, announcing that Christianity, correctly understood, was perfectly compatible with fornication, homosexuality, and abortion.

Liberal Protestant thinkers remind me of certain U.S. Supreme Court justices. The latter "find" things in the Constitution that aren't there (e.g., rights to abortion and same-sex marriage). The former "find" things in the Bible that aren't there. They claim that the Bible, rightly understood, mandates the watering down of Biblical religion and morality.

It's as if the most important teaching of the Bible is, "Don't take the Bible too seriously." In reality, of course, they find these new "truths" not in the Bible but in the anti-Christianity that happens to be flourishing at the moment, much as some German Protestants in the 1930s "found" -- mirabile dictu -- that the Bible justified Nazism.

Anybody familiar with the history of modernizing Protestantism cannot help but see this same thing going on today among many Catholics. Catholics, it is true, are moving in this direction only gradually, and this for a few reasons.

For one, they got into the game much later than Protestants did. Second, the RC Church still has bishops in places of authority, even though many bishops are reluctant, or unable, to wield their authority. Third, the Nicene or Apostles Creed is still recited at Mass, which serves as an obstacle to kicking orthodoxy completely out the door.

Orthodox Catholic morality, however, especially sexual morality, is not included in the Creeds. And so it's easier to get rid of. You get rid of it in three steps.

Step one: silence. You don't talk about it, or you very rarely talk about it. Most Catholic leaders today are shy about teaching Catholic sexual doctrine. In some cases, this is because they don't really believe in it, but in most cases it is probably because they don't want to offend folks in the pews. When it comes to the sexual behavior of gays and lesbians, our leaders know that public opinion increasingly views it as shockingly un-American or un-Christian to disapprove of homosexual sodomy.

Step two: you modify that old saying about "hate the sin, love the sinner." Instead, you love the sinner so much that don't bother mentioning the sin, for that would hurt the feelings of the well-loved sinner, and that would be a sin against Christian charity, wouldn't it? The most conspicuous example of this today is the small book by Fr. James Martin, S.J., Building a Bridge. Fr. Martin tells us he is fully orthodox. I don't doubt his sincerity, but I know, having studied the history of liberal Protestantism, where Fr. Martin, whatever his intentions may be, is leading us.

Step three: you declare that the Church will eventually, maybe 50 or 100 years from now, come around to your opinion. You argue that your apparent heresy is really nothing but premature orthodoxy.

That's the sure-fire, historically proven way to destroy the Catholic religion in America.

David Carlin is professor of sociology and philosophy at the Community College of Rhode Island, and the author of The Decline and Fall of the Catholic Church in America