Justin Welby thinks that it is a huge problem for the church in this country that it is defined by what it's against. "Young people say 'I don't want to hear about a faith that is homophobic'," he told a gathering of leaders from the Evangelical Alliance on Wednesday, many of whom have campaigned hard against gay marriage. So I asked him if he regretted that he'd voted against it.

"No," he said. "I am happy that I voted against it. It seemed to me that the bill was rewriting the nature of marriage in a way that [conflicted] with the Christian tradition, with scripture, and with understanding."

But once he'd said that he went on to say quite a lot more which showed that his thought has in fact moved on from the simplicities of the spring. First, he admitted that the church was "deeply and profoundly divided" over the issue. This is not at all what he said in the House of Lords at the time, when he claimed that all the major denominations opposed the bill. Yet there is very clear polling evidence from the Westminster Faith debates, to show that Christians, even evangelical Christians, are very conflicted about this, and the opinions of the lay members of the church much more resemble the opinions of unbelievers than they do their own leadership.

Second, he used the term "homophobia" in an honest way. There are still some evangelicals who claim it is a made-up term that refers to nothing in particular. Not so Welby. Gay marriage was, he said, an attempt to deal with issues of homophobia. "The church has not been good at dealing with it. We have implicitly and even explicitly supported [homophobia] and that demands repentance."

This is, I think, something that he sees as a command from God, rather than an adjustment to the world. That in itself is an important shift, since the only way that conservative religious attitudes will change is if they stop looking to religious conservatives like surrender.

More to the point, he now understands just how dreadful conservative Christian attitudes seem to anyone under 35. "The vast majority of people under 35 think [the church's resistance to gay marriage] is not just incomprehensible but plain wrong and wicked, and they assimilate it to racism and other horrors."

He made clear later that this attitude was found among young evangelicals as well as ordinary people, and that it was reflected in his experience as well as in public opinion polls.

Of course, this isn't really news. It is a recognition of reality much clearer and more forceful than Rowan Williams could have allowed himself, but the only possible audience for his remarks was sitting in front of him. No one outside the church cares in the slightest what its leaders say about sex. Very few people inside care either: according to the YouGov Westminster Faith Debate polls, only 2% of Anglicans take into account the views of religious leaders when making moral decisions.

Some of his evangelical audience will have heard only what he said about voting against the bill. Others will have understood what he also said about how catastrophic this attitude has become for the credibility of Christianity as a moral force in this country. But I think there is an irreversible shift of attitude under way here.