The Booker Prize-nominated English Anglican author, Francis Spufford, says he has changed his mind about same-sex marriage and is now in favour it.

Writing in the Chicago-based magazine, The Christian Century, he declares: "One major thing I have changed my mind about over the last decade is same-sex marriage. This is true of a lot of people. In fact, it is true of the majority of those who are now in favor of it.

"I should say, for clarity, that I am an English Anglican. This means that I am a member of a church (the Church of England) that still doesn't recognize marriage between two men or two women and that has had an exception carved out in English law to permit it to go on taking this position.

"But I am also a citizen of a country in which a Conservative government moved un­ambiguously to make the change nearly a decade ago and where same-sex marriage has settled swiftly into place as a new normal. That is, same-sex marriage is still contentious in my church but not much in my society."

Spufford is honest enough to admit that his change of mind on this is a major shift in the historic Christian understanding on the issue. He is also honest enough to admit that those advocating for the Church to bless same-sex marriage as an institution are effectively arguing that the Apostle Paul was wrong to condemn practising homosexuality in Romans 1.

"I value the work that's been done to nuance and contextualize the Pauline condemnations in Romans. I see that there is a specific force to Paul condemning 'men who lie with men' in the context of a slave-owning rape culture where high-status men felt entitled to help themselves to human flesh of every variety," he writes.

He agrees that Paul "wouldn't have had before his mind's eye any models at all for relationships between men, or between women, that were marked by mutuality".

But he goes on to say that he is not convinced "that the rule against gay sex was therefore never really intended to apply to sex between loving equals".

"I don't think we're really saying that Paul has been misunderstood for two millennia. I think we're saying that Paul was wrong," he writes.

Why does Spufford think it is right for Christians to say Paul was wrong about gay sex? In essence, because he believes Christians have had to say Paul was wrong about slavery.

"The abolitionist movements of the 18th and 19th centuries could not point to an unambiguous and established rule against slavery as such. Instead they had to manufacture, from equally scriptural but not legal materials, a new moral consensus," he contends.

"They had to discover a fundamental incompatibility between the prophetic thrust of scripture toward justice and mercy and the idea of humans owning humans, between the Bible's fundamental picture of personhood and the idea of humans as commodities. And on the strength of that, they had to learn to set aside Paul sending Onesimus back to his master in Philemon.

"None of us doubt now that the Holy Spirit was guiding this particular work of realization. This innovation, we are all sure, represents a necessary development of what was latent in our scripture and our doctrine all along. The scandal here is not the change but that it did not come sooner."

So that's it then. Those of us who believe that marriage is an exclusively heterosexual institution need to see the light and get up with the programme. Otherwise, we risk being on a moral par with 18th or 19th Century 'Christian' slave-drivers.

But not so fast, Mr Spufford. Unfortunately, you have misunderstood Pauline theology.

Paul never said slavery was a good thing. In his eyes, it was an institution that belonged to a fallen world, an evil-ridden realm in bondage to decay and awaiting its eternal redemption by the Lord God Almighty's returning Christ.

In this temporal, fallen world, the most important aspect of a Christian's identity is his or her eternal union with humanity's Saviour, the Lord Jesus Christ. Our social status is secondary but not insignificant. If a Christian person can gain freedom from slavery, he or she should do so because slavery, unlike marriage, attaches to the Fall of mankind.

It is within that doctrinal framework that Paul's advice to Christian slaves in 1 Corinthians should be understood:

"Were you a slave when you were called (to eternal salvation through the gospel of Christ)? Don't let it trouble you – although if you can gain your freedom, do so. For he who was a slave when he was called by the Lord is the Lord's freedman; similarly, he who was a free man when he was called is the Lord's slave. You were bought at a price; do not become slaves of men" (1 Corinthians 7v21-23 – NIV).

Unlike slavery which attaches to the Fall, marriage attaches to God's good Creation and therefore Christians should not break their marriages even if they are married to unbelievers, as Paul also teaches in 1 Corinthians:

"If any brother has a wife who is not a believer and she is willing to live with him, he must not divorce her. And if a woman has a husband who is not a believer and he is willing to live with her, she must not divorce him" (1 Corinthians 7v12b-13).

Because marriage belongs to Creation whereas slavery belongs to the Fall, to put them both up for revision, as Spufford does, is like comparing a breath of fresh air to an attack of lumbago.

As for Spufford's cursory treatment of Paul's letter to Philemon, it is important to be clear that Paul didn't just send Onesimus back to his master; he also pleaded with Philemon from the basis of their shared Christian worldview:

"I am sending him - who is my very heart - back to you. I would have liked to keep him with me so that he could take your place in helping me while I am in chains for the gospel. But I did not want to do anything without your consent, so that any favour you do would not seem forced but would be voluntary. Perhaps the reason he was separated from you for a little while was that you might have him back forever— no longer as a slave, but better than a slave, as a dear brother. He is very dear to me but even dearer to you, both as a fellow man and as a brother in the Lord" (Philemon v12-16).

Again, Paul's view of a Christian person's eternal identity as the primary aspect rather than their temporal social status is strongly present here. It is Paul's teaching about Christian identity that led Christian slave owners in the Roman Empire to want to liberate their slaves but they knew they had to do it legally rather than inciting a slave revolt.

Julian Mann is a former Church of England vicar, now an evangelical journalist based in Morecambe, Lancashire.