Last Thursday over six hundred hundred bishops moved slowly and silently behind a banner telling G6 governments to "Keep the Promise, Halve Poverty by 2015".

The Archbishop of Canterbury led the walk past Downing Street and the Houses of Parliament to Lambeth Palace. In their hands the Bishops carried Poverty and Justice Bibles, a recent publication highlighting more than 2,000 biblical passages dealing with issues of poverty and injustice.

The event was a good example of the church engaging in politics. In an open letter to the Prime Minister, the Archbishop of Canterbury wrote:

"In our human family the continuing consequence of failure is the death of millions of women and children from childbirth and preventable childhood diseases. Millions more have hopes for sustainable life diminished because of poverty-fuelled conflict, lack of education and gender inequality, all of which undermine a new generation's liberation from HIV."

Gordon Brown hailed the march as "the greatest public demonstration of faith" Britain has ever seen."‘100 years is too long to wait for justice and that is why we must act now," he added.

The Prime Minister continued: "You have sent a symbol, a very clear message with rising force that poverty can be eradicated, poverty must be eradicated and if we all work together for change poverty will be eradicated." 

The bishops were equally enthusiastic in their response to Gordon Brown. In a mid-Lambeth Conference Letter to the Diocese of Durham, the Bishop of Durham, Rt Revd Tom Wright, commented that "the centrepiece of [the event] was Gordon Brown's remarkable speech, passionate, without notes, all his facts and figures in place and beautifully balanced between theory, personal stories, appeal, etc. Everyone else from around the world was really quite jealous of us having such a man as our leader."

Mr Brown must have wished that members of his own party could be so generous in their praise.

In the comment pages of the national media, the intervention of the worldwide Anglican Communion was universally welcomed. The Daily Telegraph described the spectacle as a "true act of Christianity", but the response of commentators to the bishops' protest stands in stark contrast with their response to interventions on other more controversial topics.

The legitimacy of allowing religious conviction to enter the public square appears remarkably flexible. Why is it acceptable for religious leaders to speak out on poverty but not on, for example, bioethics?

Why are religious leaders applauded for calling on the G8 to keep their promise and halve poverty by 2015 but condemned when they criticise politicians for applying utilitarian ethics elsewhere?

The contradiction appears illogical. Is the contribution of religious leaders only to be welcomed when it coincides with the considered views of journalists and other popular commentators?

We can't pick and choose. We must either welcome or oppose religious contributions to public debate. If we do the first (and the evidence of last Thursday is that we should), then we must encourage the contributions of religious leaders to all debates, whether or not we agree with them (perhaps especially if we don't). By excluding them, we not only diminish those debates but also ourselves.

This article first appeared in the Daily Telegraph Ways and Means blog.