Dismissing St Thomas Aquinas with his customary succinctness, Bertrand Russell said there was “little of the true philosophic spirit” in the Catholic church’s greatest philosopher. It was not that he wasn’t clever, but that he lacked the willingness to follow an argument wherever it led. Catholic dogma determined in advance where he could and could not go.

Aquinas “already knows the truth; it is declared in the Catholic faith. If he can find apparently rational arguments for some parts of the faith, so much the better; if he cannot, he need only fall back on revelation. The finding of arguments for a conclusion already given in advance is not philosophy, but special pleading.”

Anti-Catholicism is still a residual force in Britain and a great mistake of secularists who fall into the prejudice is to imagine that the church’s dogmas are as dumb as the blabbering of the worst type of knuckle-dragging, know-nothing Protestant fundamentalist. Reading the pope’s encyclical on the care of our common home ought to provide an antidote. It shows, if nothing else, why apparently intelligent men and women can lose themselves in the richness of Catholic theology.

The encyclical is plainly written – Francis examines our throwaway society and declares: “The Earth, our home, is beginning to look more and more like an immense pile of filth.” It emphasises the interconnectedness of the natural world and is properly appalled that humanity is presiding over a mass extinction of species. Francis shows a concern for the poor that would shame most rich leftwingers, let alone their conservative opponents. And like his recent predecessors he is comfortable with modern science. Even if you think, as I do, that there is no God and the pope should grow up, you should admit that there is grandeur in this view of life, to take Darwin’s words wildly out of context, and a moral vigour too.

But it won’t wash.

When he is forced to choose between intellectual honesty and dogma, the pope chooses dogma without hesitation. Like Aquinas, he descends into special pleading, without admitting to the reader or perhaps to himself that he is rigging the debate.

You do not need to see humanity as a cancer killing mother Gaia to accept that a world of 7 billion is overpopulated and will become grossly overcrowded if humanity reaches 9 billion in 2050. Every extra person on the planet increases carbon emissions and the demands on scarce resources. Their need for energy may outrun the best efforts to build clean carbon-free alternatives to oil and coal, and force us to wrench out fossil fuels,which should be left in the ground. As for the other species we share our planet with, humanity will, inevitably, grow into what little space they have left. Put like this, birth control is an environmental necessity.

Argue that point, however, and you are hit with a response that can sound plausible. The children poor people have in poor countries consume so little that they are almost an irrelevance from an ecological point of view. True as this may be, no one can act on this insight without turning into a monster. A sincere believer would have to insist that the EU closed its borders to migrants and refugees, not out of a nativist desire to protect ethnic purity but an environmentalist desire to stop poor immigrants becoming rich producers of greenhouse gases. I have never heard the deepest of greens cry: “Send them back”, although Mother Teresa, accurately described by the much missed Christopher Hitchens as “a fanatic, a fundamentalist and a fraud”, did patronise the Indian poor by telling them their suffering was a gift from God .

The pope does not say that the poor must stay poor to show their gratitude to the almighty or for the sake of the environment. Rather, he ducks the question of what will happen as the ever-expanding populations of poor countries grow richer. Demand the promotion of birth control – not abortion or eugenics, just contraception – and you are “refusing to face” the world’s unequal distribution of wealth, he writes. End “the extreme and selective consumerism” of the rich world and – eureka! –“demographic growth is fully compatible with an integral and shared development”.

Everything about his argument is slippery. Even if rich countries are prepared to redistribute wealth to poor countries, I have never met a secular campaigner against poverty who does not believe that educating women and giving them control of their fertility is the best way to reduce poverty. More pertinently, the pope is not against birth control because he believes in the redistribution of wealth but because Catholic teaching says he must damn it. For the little it is worth, which is next to nothing in my view, Francis has scriptural authority on his side. When Onan refused to impregnate his dead brother’s wife and “spilled his seed on the ground”, the Lord made him pay the ultimate price for pleasuring himself and slew him. The fathers of the church were only slightly less harsh. Clement of Alexandria said: “Seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted”, and Augustine thought that sex “for the sake of lust” rather than procreation led to sterility.

Given his bad faith, given, too, that he had the power to reduce poverty by changing the church’s theology and refused to do it, the mystery is why so many liberals believe the pope is not a Catholic. They denounced Pope Benedict, because he was in the Hitler Youth, as if any young German of his age had a choice in the matter. But my old friend and comrade Jonathan Freedland and many others hailed Francis as the “new hero of the left”, even though he has never given the smallest indication that he intends to change one syllable of Catholic teaching. As the baffled editor of the Catholic Herald put it, spin had led liberals to believe in a “fantasy Francis” .

The credulity of believers can be bad enough but the credulity of liberals can be worse. If you don’t wish to join the gormless herd, by all means welcome the pope’s commitment to the poor and to preserving the environment. But you must accept that if you believe in giving women the opportunity to expand their minds and control their bodies, the Vatican is against you – as it always was.