There’s an obscure little verse in the Book of Proverbs which says “Do not move the ancient boundary which your fathers have set” (Proverbs 22:28). Whilst it refers to physical landmarks and God’s requirement for integrity in the ownership of land and property, it would, I think, be entirely reasonable in today’s context to apply the verse to the longstanding moral and spiritual boundaries we have inherited through many generations.

More than one moral boundary stone is currently at risk. Not only has Prime Minister David Cameron committed himself to changing the traditional heterosexual definition of marriage, but also over the past week we were presented – from a different quarter - with the case for legitimising infanticide. An article in the Journal of Medical Ethics, co-authored by a research associate at Oxford, hypothesised that “Medics should be able to kill newborn babies if they are disabled, too expensive or simply unwanted”. The argument put forward by Dr Francesca Minerva and Dr Alberto Giubilini was that neither foetuses nor newborns have the same moral status as actual persons.

In one sense, there is an inescapable logic in their argument. If liberal ethicists consider that even late-term abortion is legitimate, what essential difference is there between a theoretical pre-natal termination at 39 weeks or a post-natal termination a week later? The location of the baby, whether in the womb or in the cot is irrelevant if the infant in either locality is considered to be no more than a “potential person”.

But here, as with the issues regarding marriage, we are faced with the intention of shifting ancient moral and spiritual boundaries. What many people failed to recognise is that once a boundary has shifted once, there is every possibility that it will shift again. By the time such recognition dawns, it is too late to reverse the process. Opening the door to abortion on demand has cheapened life to the extent that infanticide, something unthinkable fifty years ago, can now be seriously aired as a potential way of getting rid of inconvenient babies. Did nobody see that coming?

Similarly, the Prime Minister’s enthusiasm to follow other European and Western nations in legalising same-sex marriage will inevitably then result in pressure for further boundary shifts by opening the door to legal polygamy (or polyamory). As Cardinal Keith O’Brien shrewdly observed “If marriage can be redefined so that it no longer means a man and a woman but two men or two women, why stop there? Why not allow three men or a woman and two men to constitute a marriage?”

Arguments will be made that just as not all married couples have children, likewise not all marriages are monogamous. It has already been claimed that prohibitions on plural marriage breaches people’s civil rights. If their lifestyles are “not harming anyone” then there should not be “discriminatory” legal restrictions. Indeed, the argument goes, anyone’s sexual preferences that take place behind closed doors among consenting adults ought to be validated. Personal autonomy now trumps long-established moral boundary stones. And if every sexual orientation has a right to fashion its own definition of marriage, then you can be certain that David Cameron’s ‘reform’ of marriage will not be the last. Can nobody see that coming?

Many church ministers are familiar with what is known as the principle of “moving the piano six inches a month” – a euphemism which describes the practice of introducing change in minor adjustments so imperceptibly that traditional congregations remain oblivious to the change until the necessary goals have been achieved. Advocates of gay marriage employed a similar practice in Holland, where the campaign for same-sex marriage was conducted on a “small-step strategy”, and each small increment of recognition or acceptance created an impetus for further steps.

Britain has done likewise. In 2005 the British Government introduced the civil partnerships legislation, which essentially legislated for same-sex marriage in all but name. At the time the Government claimed that it had no intention of introducing same-sex marriage, and sought to reassure the Church of England’s House of Bishops that civil partnerships were not a form of marriage. More recently, senior Labour politicians admitted that, in fact, same-sex marriage was the intention all along. And no, the Church apparently didn’t see that coming.

The ancient boundary stones are unquestionably being moved and even if infanticide seems inconceivable today, or the prospect of polygamy seems currently outlandish, just remember that that was what was said about abortion and legalising same-sex marriage only a few years back. The worrying thing is that slippery slopes are not visible to a society that has gone blind.