Throughout the course of Christian history there have been many expressions of this view of marriage and the family, but I want to examine just one. His is a name difficult to avoid on the topic: St Augustine, the great Bishop of Hippo in North Africa. Augustine saw marriage first of all as the coming together of man and woman for the sake of children, but also for the sake of the security of the partners. This is what you might call the contractual view of marriage. It needed to be understood in a lifelong sense because, apart from anything else, the human child takes a long time to grow up.

But Augustine didn't stop there. He went on to speak of the commitment that is necessary — contract is not enough — so that we do not use one another simply as a means to our own selfish ends, but commit ourselves to the other as a person. Augustine also spoke of the sacramental bond, which means that you are now not talking about two but one — the unity that is created by the complementarity of man and woman. It is a unity that arises out of similarity and difference. There has to be another in the marriage so that we can come together in this particular way for the common good, for children, and for one's own fulfilment.

Augustine has remained salient in nearly all the thinking about marriage, certainly in the Christian church but indeed well beyond that — for example, in the Enlightenment. 

Let's take three typical thinkers of the Enlightenment. John Locke emphasised the importance of the contractual side of marriage and particularly the contract that is undertaken for the birth and nurture of children. But the implication is that the contract might not last beyond the growing up of the children. That's the weakness in Locke's position, whereas we would have to say, relying on Augustine, that the contract is not just for the sake of the children but also for the security of the partners. What happens when you have brought up the children and then you are abandoned? This is not an unfamiliar story these days.

Immanuel Kant, on the other hand, developed St Augustine's emphasis on commitment into what he called the unbreakable promise — when you undertake a vow it is then your duty to keep that vow. For him there is no duty higher than the keeping of a promise. 

Contract and commitment in this sense of duty, of unbreakable promise, are important elements of marriage, but it was Hegel, another great Enlightenment figure, who talked about the mystical union. Here we are coming very close to the Augustinian idea of the sacramental bond. For Hegel the differences that exist between the two persons are overcome so that there is a real unity of thought, direction and destiny in the marriage. Although in the Christian tradition, marriage between the baptised is thought of as sacramental because it is a sign of unity between Christ and his church, Hegel extends this to marriage in the natural sense. This is reflected to some extent in the Anglican emphasis on marriage as a creation ordinance.

Augustine laid the groundwork and Enlightenment philosophers developed it, but the sad fact is that today there are threats to all of their ways of understanding marriage. Even if you think of it as mere contract, what has happened to that? Since the arrival of no-fault divorce without consent, is marriage even a contract any longer? What kind of contract is marriage now if you can get out of it more easily than your mortgage? The so-called reform of divorce laws has had a destructive impact on marriage.