It was witty of the Cambridge Union to set up the debate as they did: a priest (me) leading the case against an established church, and a card-carrying atheist (Stephen Fry) leading the case for. But more than just witty, for it disrupted the familiar assumption that the establishment of the Church of England is simply a question of the church desperately clinging on to its place of historic privilege.

Fry is one of those atheists who like the Church of England because they think of it as full of kindly priests who don’t believe in anything very much. As such, he argues, it acts as a kind of bulwark against religious fanaticism. At its core is a misty-eyed, Downton Abbey-esque idea of the C of E: smiling vicars on bikes, evensong etc. Having been to the same public school as Fry, I know where this nonsense comes from. But, he argues, through the C of E the English get the tiniest little bit of religion (which they can sleep through) and are thereby inoculated for life against the full-blown and often violent version of the disease.

Even this was overly generous coming from a man fresh from nuptials, the possibility of which the C of E did so much lobbying to oppose. And was a point well made in the debate by my allies at the British Humanist Association. The church’s behind-the-scenes influence over the whole legislative process on gay marriage was clearly disproportionate to any support its views had in the country at large.

But my temporary alliance with the BHA was not because I share any of their hostility to religion. Rather, because I think establishment weakens the church and turns clerics into fawning Jeeves-like courtiers who prefer dressing up to speaking out. Take, for example, the offensive nonsense of Westminster Abbey lowering their flag at half mast out of respect for a king who presided over a regime with one of the worst human rights records in the world and which beheads people with swords on a similar scale to Islamic State, and where conversion to Christianity remains a capital offence. Why did they do this? Because, as the abbey helpfully explained: “It is at half-mast because the government has decided to fly their flags at half-mast today.” And that is precisely the problem with the whole mentality of establishment.

This has been a persistent issue from the moment Christianity became popular with the ruling classes. Soon after the Emperor Constantine superstitiously decided that Christianity helped him win battles and so converted the Roman Empire to the sign of the cross, Christian courtiers have been helpfully rowing back on all that stuff Jesus said about not fighting and giving all your money away – neither of which were particularly attractive propositions for a man who lived in a palace and who headed up one of the largest war machines the world had ever seen. And, surprise, surprise: Constantine insisted on appointing his own bishops.

This is the great heresy of an established church – it ends up forgetting who the boss is. And this forgetfulness has stalked the church throughout centuries of what came to be called Christendom – that pernicious self-supporting symbiosis of church and state. My hope is that as Christendom continues to fall apart, so the church may be able to recapture some of its pre-Constantinian vitality and radicalism. It won’t be as big or as strong, at least not in worldly terms. But that was never part of the promise of the Gospel. And if the church is not vigorous enough to stand on its own two feet without a state-sanctioned life-support system, it deserves to pass away into obscurity.

In the end, my side won the debate – ayes 265, noes 86, abstentions 90 – registering a very tidy 20% swing in our favour. I had thought the Stephen Fry effect might swing it for the supporters of establishment. But in the end even he walked through the abstention door. I guess this is precisely his view of the C of E – that we are life’s natural (but kindly) abstainers. And that’s something the church must change: change or die.

@giles_fraser