The Catholic Church was once known for the outstanding intellectual capacity of its senior figures. Scholars such as Aquinas, St Augustine and St Jerome could all unpick a subject with dazzling philosophical dexterity. As doyens of rhetorical theory, when they made a point it was powerful, compelling, steeped in learning and hard to rebut. Even an atheist like me finds much to admire in Catholicism's illustrious scholarly heritage. So, it's a bit of shame that the modern church is represented by, on this occasion, Cardinal Keith O'Brien.

He seems to possess the philosophical subtlety of a pot plant, and the communication skills of a cringe-inducing Ricky Gervais character, capable only of digging himself deeper and deeper into the quagmire of outrage with the utterance of every idiotic vowel. I am no expert, but here is my advice to the blundering Cardinal. If you are trying to appeal to traditional values, and draw moderates to your side of the argument, it's probably best to leave slavery out of it. Slavery is one of those issues that is morally repugnant to pretty much everyone in the known universe. Slavery makes incest look like a heart-warming Disney movie by comparison.

In fairness, the Cardinal's point (ironically, made during a radio interview intended to undo the earlier damage from a previous rant) was that legalizing homosexual marriage is the moral equivalent of bringing back slavery. Okay, so at least he wasn't calling for the return of slavery - it's a good start. Unfortunately, that is pretty much the only positive one could draw from such a self-destructive argument.

Institutionalised slavery has been with us for thousands of years, dating back to the ancient Egyptians and beyond, but it really reached its disgusting zenith between the 16th and 19th centuries, when the so-called Atlantic Triangular Trade saw millions of Africans shipped to the Americas to work on plantations. Some of the biggest slave owners were... yes, you guessed it... churchmen. Alarmingly, both the Catholic and Anglican Church put up staunch resistance to the call for abolition, citing the Bible's lack of condemnation for slavery as justification of its acceptable morality.

So, you can perhaps understand how Cardinal O'Brien's comments are so spectacularly damaging to his own argument. In his exposition of traditional Christian morality, he is invoking the same religious text used by pro-slavery lobbyists two hundred years ago, but is now on the opposite side. Instead of defending slavery, he is using it as a comparable evil to remind us of the horrors of homosexual marriage. In this process, and in an exquisitely fumbling way, he has proven that...

a) What is acceptable practise in the Bible (slavery) is now deemed universally immoral
b) Therefore, what is immoral in the Bible (homosexuality), is also open to revisionist thinking
c) Therefore, morality is not a fixed set of Biblical laws, but a shifting intellectual framework that evolves over time to reflect each society
d) Ergo, homosexual marriage cannot be called 'immoral' unless it is a value shared by the majority.
e) A recent poll showed UK Christians to be 61% in favour of gay marriage. This does not even include non-Christians.

Cardinal O'Brien may try to claim that the Bible alone dictates morality, but he is in for a tricky ride. May I remind you of the ban on haircuts and shaving your beard (Leviticus 19:27), the rule about not eating shellfish (Leviticus 11:10), the thing about women not being allowed to wear any expensive clothes or jewellery (Timothy 2:9), the ban on polyester clothes (Leviticus 19:19) and the fact that any descendent of an illegitimate person is not allowed in a church, even after 10 generations (Deuteronomy 23:3). Clearly, things change. These are rules that have been ignored and forgotten. This makes it logically impossible to say the Bible is unimpeachable divine law - if you are going to pick and choose from its teachings, then it becomes solely a philosophical handbook.

Cardinal O'Brien also stated that giving equal rights to homosexuals is "changing the whole notion of what marriage and what a family is..." As a historian, this is where I get a bit pedantic. Marriage is much older than Christianity, and has taken many forms. The ancient Egyptians simply required a couple to move in together, while under Roman law you could be married purely by saying "yes" to a proposal. Spartan marriages involved the bride having her head shaved, cross-dressing like a man, and being kidnapped and deflowered by her groom, followed immediately by the man's disappearance for 10 years' military service. She might see him again when he turned 30!

As for Christian marriage, it has already gone through huge changes... twice. For starters, a literal Biblical marriage is a polygamous one (Exodus 21:10) that allows a husband to stone his bride to death if she is not a virgin on their wedding night (Deuteronomy 22:22). I'm sure Cardinal O'Brien is very glad the whole notion of marriage is not as it once was. Secondly, it was barely until the 17th century that marriages were vaguely religious or even conducted in churches.

We must remember, the romantic notion of marrying for love is so new, it's still got the cellophane wrapper on it. Marriages were traditionally economic and political contracts between families. In the Early Middle Ages, money was exchanged between grooms, fathers-of-the-bride and wives to be, in much the same way that a transferred-footballer picks up a separate signing-on fee in addition to the clubs agreeing a price for him. The Saxons and Vikings were surprisingly liberal in their attitudes towards women, and the bride was given a cash sum called the 'morning gift' by her groom that she got to keep if things didn't work out. Intriguingly, if a Saxon woman were kidnapped, her husband was legally obligated to try and buy her back from slavery before seeking remarriage. His wife, however, had no such obligation if the roles were reversed.

So why were Saxon and Viking wives so well rewarded? It is because marriage had a powerful social function. Wives in this society were known as 'Peace-Weavers' - they were there to solidify unity between often fractious neighbours, and their marital role was one of applied political diplomacy, aiming to soften the hostile harshness of male aggression. Marriage was a peace treaty, and wives were the Ban Ki-moons of their village. Alas, it was the Normans, and their aggressively misogynistic brand of Christianity, who abandoned this and turned brides into ciphers for property. Women's legal rights were almost entirely stripped away, and wives became political assets for boosting one's property portfolio, and ensuring legitimate heirs. That is not to say there was no romantic love. William the Conqueror was devoted to Matilda... though he proposed to her by kicking her repeatedly, and throwing her into the mud until she said yes. You don't see that in many rom-coms.

Throughout the next few centuries, marriages were largely practical alliances devoid of much sentiment. Men often sought women for the company, cleaning and cooking, and women sought the stability and respectability of a sensible union. Such pragmatism was exemplified in the wedding service itself. Until 1754, a British marriage ceremony could be conducted anywhere, in front of any public official. Bizarrely, the most common place in London was the Fleet prison. There were more than 300 weddings a week, accounting for 50% of the capital's marriages, in this overflowing gaol. In 1754, just hours before the law changed, Reverend Dr Keith found himself conducting 100 services in a single day! Such romance...

Cardinal O'Brien seems to be yearning for tradition, but the halcyon sort that doesn't actually extend very far into the past. Admittedly, I cannot make a direct rebuttal of his case against gay marriage, because there is scant evidence of same-sex marriage in history - Roman emperors Nero and Elagabalus were known to have married male slaves, but these were not legally valid. However, the issue at stake is whether marriage is an immutable concept, and I believe history shows that it isn't. The meaning of marriage, and the way it has been performed, has changed tremendously over thousands of years. If the Cardinal wants tradition, then what could be more traditional that continuing the legacy of updating marriage to reflect our modern and humane society, where loving partners of any gender, creed or identity can choose to commit themselves in loving support of each another?

That said, I'd love to bring back the Viking wedding ceremony, which culminated in an insult competition, and then usually a fight. Some things should never change!

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