To understand the extent and the growth of atheism in America, it is important that we attend not just to atheism properly speaking but also to what may be called “semi-atheism,” by which I mean those not-quite-atheistic beliefs that incline toward atheism, beliefs that at least “tend” in the direction of atheism. Let me offer, by way of explanation, a necessarily incomplete list.

First on the list is of course atheism itself. It comes in two forms: personal atheism and absolute atheism. The first of these is found in the person who says, “I myself don’t believe in God.” But this person, filled with a spirit of tolerance (or, at all events, indifference) does not object if you believe in God. The absolute atheist, by contrast, flatly denies that God exists, and he clearly would like to make sure that you too should not believe in the existence of God, for the same reason that you should not believe in elves and fairies – because there are no such things.

Next on the list is agnosticism. In theory, the agnostic stands at a midpoint between belief in God and disbelief. This theoretical agnostic says that the chance that God exists is approximately 50/50. Now some agnostics say this. But in practice, if a present-day American tells you he’s an agnostic, he’s likely to be a virtual atheist. That is, while he grants that nobody can be absolutely certain that God is non-existent, he feels that the chance of God’s existence is about one out of a thousand, or a million, or ten million.

Besides, there’s something rather rude about proclaiming oneself an outright atheist; it gives offense to one’s believing neighbors. An atheist with good manners prefers to describe himself as an agnostic. The typical agnostic lives his life as if there is no God: he doesn’t say his prayers, he doesn’t go to church (except for weddings and funerals), and he certainly doesn’t regard the rules of morality as God-made commandments.

Agnosticism, like atheism, comes in two varieties: (a) the personal agnostic, who says, “I myself doubt the existence of God, but I have no objection if you believe;” and (b) the absolute agnostic, who insists that the question of God’s existence, given the limits of our human intellect, is an unanswerable question. And you shouldn’t try to answer it.

Third on the list is what may be called modern deism. This includes people who, starting with the Judeo-Christian idea of God, have stripped it down to its bare-bones minimum, getting rid of all non-essentials. And what are the non-essentials? Well, everything that seems implausible to a thoroughly secularized modern (or postmodern) mind. Jesus was a good and wise man, but certainly not God incarnate.

He was an ancient version of those two martyred moderns, Mahatma Gandhi and John Lennon. God did not inspire the writers of the Bible. God did not found the Catholic Church or any other Christian church. From God’s point of view, in fact, churches are quite needless institutions; indeed, they are harmful institutions.

These deists are “spiritual but not religious” – the translation of which goes like this: “I am religious in a very private and personal way, but I don’t belong to any organized religion.” Some aspects of Christian morality are good from the deistic point of view: for example, love and tolerance (they always imagine that Jesus was a champion of tolerance). Other aspects are hopelessly outdated: for example, Christian sexual morality. And who or what is God? God is a mysterious yet friendly force that lies behind the visible world.

Fourth on the list is the “liberal” Christian. Liberal believers are those who hold that the dogmatic element of Christianity is either of no importance or of minimal importance. The important thing, indeed the all-important thing, about Christianity is its moral element. It doesn’t matter what you believe; it matters how you feel and how you behave.

This being so, liberal believers are free to discard any and all traditional Christian doctrines they find inconvenient or implausible or not in step with the superior wisdom of affluent 21st-century countries. And although liberals believe that moral goodness is the essence of Christianity, their freedom from dogma allows them to redefine what counts as moral goodness and badness.

As a consequence, it is not unusual to find liberal Christians who not only reject almost every part of the Nicene Creed (except for “was crucified under Pontius Pilate”) but who also give heartfelt moral approval to fornication, abortion, and same-sex marriage. The more extreme forms of liberal religion are barely distinguishable from the deism mentioned above except for two things: liberals like to call themselves Christians, and they like to belong to a church.

It’s an easy slide from one of these to the next – from liberal religion to deism, from deism to agnosticism, and from agnosticism to full-blown atheism. It is the slipperiest slope. And so, when Christians decide to “liberalize” their faith, there’s a good chance that their children or grandchildren will end up in atheism.

“But,” some may object, “if the slope is slippery and the steps are small, why can’t the progress go in the other direction? Why can’t we slide from atheism to agnosticism, from agnosticism to deism, from deism to liberal Christianity, and from liberal religion to full-fledged Christian orthodoxy?”

Well, we can. And some do. But nowadays they are the exception, not the rule. Much depends on how the slippery slope is tilted. And in the USA and other developed countries today it is tilted in the downhill direction of atheism. Atheism, along with various kinds of semi-atheism, dominates what may be called the “cultural command posts” (i.e., media, entertainment industry, and higher education).

As a consequence, it has a much stronger gravitational pull than does traditional Christianity. And overcoming that tilt requires not only renewed vigor in the churches but a cultural reversal only rarely seen in human history.