In a recent The Rest is Politics podcast, hosted by Alastair Campbell and Rory Stewart, the Archbishop of Canterbury, Justin Welby, informed us that he has been on a journey. From being a clear advocate of conservative Evangelical belief, affirming sex only within traditional marriage, he tells us that, as a result of much prayer and theological reflection, he now embraces a position that sees sex as permissible in any stable relationship, and whether heterosexual or gay makes no difference.

He said, to quote: "all sexual activity should be within a committed relationship, whether it's straight or gay. In other words, we're not giving up on the idea that sex is within marriage or civil partnership. We've put forward a proposal that where people have been through a civil partnership or a same-sex marriage, equal marriage, under the 2014 Act, they should be able to come along to their local, to a church, and have a service of blessing for them in their lives together" (emphasis added).

In other words, and lest there be any doubt, what the leader of the Anglican Communion is saying is that the traditional Christian view of marriage as between one man and one woman for life is pretty much redundant and that all sex is fine – provided only that it takes place within what he defines as a stable (albeit perhaps temporary) relationship. And, as added insult to injury, he then goes on to claim that this does not and will not affect the Church's stance on same-sex marriage, which currently permits a service of blessing for same sex couples but draws the line at marriage.

Such a position is not just theologically indefensible, but logically irreconcilable. For the worldwide head of the Anglican Church to say that all sex is fine – whether inside or outside marriage, and whether gay or straight – is inevitably, by extension, to endorse same-sex marriage. But he would do well to remember that in Scripture, sex outside marriage and homosexuality are both branded anathema.

So what does the Archbishop's rather rambling announcement say about his commitment to Christian belief? Is there any doctrine he now holds to be sacrosanct – or is the truth actually that, deep down, he no longer believes? Has his 'God' in fact become no more than a sort of squidgy comfort blanket and legitimisation for social action? And was Christ's sacrifice to redeem mankind from sin, objectively commendable though it might appear, ultimately pointless?

Such a position is not, and never can be, right. Christianity is not founded on feelings or being 'nice' to people, but on the revelation of God and of His Son, Jesus Christ, as set down in the Bible. No one who holds otherwise is fit to hold office in the Church.

So what does the Bible say exactly? From Genesis onwards, Scripture is clear that marriage is between one man and one woman for life, a position taken over and affirmed by Jesus himself. See, for example, Matt 15:5, "For this reason, a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh."

Similarly, though interpreted by liberal activists as a cultural and time-bound anachronism, the Bible is unequivocal in its condemnation of homosexuality. Leviticus 18:22 clearly states, "Do not have sexual relations with a man as one does with a woman; that is detestable." While chapter 20, equally trenchant in its condemnation, says that all people caught in such behaviour, both male and female, shall be put to death.

Strong stuff, and no one would wish to see such a sentence carried out today, but it shows clearly how such behaviour was regarded and the weight of condemnation it attracted. Similarly in the New Testament, the apostle Paul was unequivocal in his denunciation of homosexuality and lesbianism, labelling such practices sinful and degrading, and leading to damnation: e.g. Romans 1:26-27.

It would seem, however, that after 'mature spiritual reflection', Archbishop Welby has decided that the Bible is wrong ... or at least never meant what it said. Perhaps he imagines that God too is on a learning curve, and has now recognised that mankind has come of age, so that it is no longer God who teaches mankind how to behave, but vice versa, and we need no longer be bound by the moral strictures of yesteryear. But where, one wonders, will all this stop? If it's possible so easily to jettison one commandment, why hang on to the rest?

It would seem that the Archbishop, by his own admission, is a convert – it's just a pity that his conversion has been to a faith other than the one he has been entrusted to lead.