My text this morning comes from the book of the prophet Jeremiah, chapter 53, beginning at the first verse: "And it came to pass that Jeremiah entereth the territory of Ba-rack and stirreth up trouble and gnashing of teeth among the tribes. And verily Hillary, queen of the Monicans, see-eth it and thanketh the Lord for his merciful kindness, saying, At last I have found my Mr Wright'."

A storm of biblical proportions is currently raging across America. At the core of the storm is an incendiary black preacher, a potential American President of mixed race, and an ambitious and rather desperate white woman who will do anything to thwart her Democratic rival for the White House. The black preacher is turning into the black(ish) candidate's nightmare and the white woman's salvation. We live in strange times.

From the eyrie of the electronic croft in Orkney I want to look at the scene from a theological perspective. What is the theology that sustains the Reverend Jeremiah A Wright Jr, pastor to presidential hopeful Barack Obama?

I watched an hour-long conversation between Wright and veteran broadcaster Bill Moyers and saw longer versions of the sermons from which the soundbites that have horrified America were ripped. Far from living up to the current caricature of an ignorant fundamentalist who hates America, Jeremiah Wright is a highly educated, sophisticated man. His Chicago congregation, which has grown in numbers from 87 to more than 6000 during his 35-year ministry, has developed many outreach projects in terms of feeding the hungry, helping the poor, sitting with the drug addicts, defending gays and speaking out against injustice.

Having studied black theology at Duke University in North Carolina, I immediately recognised Wright's themes. Western theology for centuries was dominated by white affluent males, many of them in favour of slavery.

The emerging black theologians of the 1960s and 1970s turned traditional theology on its head, re-envisaging Christianity from the point of view of the oppressed.

In New York with George MacLeod in 1985, I went to a service in a black Baptist Church in East Harlem. The singing was exuberant and the preaching was electrifying. Black congregations expect their preachers to represent the biblical prophetic tradition, speaking truth to power. In an America awash with guns, this can be dangerous, as evidenced by the assassination of Martin Luther King. Wright himself has had death threats.

So what about the pastor's most notorious passage - shown on an almost permanent loop on American TV - about God damning America? In the sermon, Wright said God does not bless everything.

With rhetorical flourish he went on: "When it came to treating citizens of African descent fairly, America failed. She put them in chains kept them out of their racist bastions of higher education and locked them into positions of hopelessness and helplessness.

"The government gives them drugs, builds bigger prisons, passes a three-strike law and then wants us to sing God bless America? No, no, no. Not God bless America; God damn America! That's in the Bible, for killing innocent people. God damn America for treating her citizens as less than human. God damn America as long as she keeps trying to act like she is God and she is supreme."

Wright has argued - correctly - that the preacher's job is not to tell people what they want to hear, but to be truthful. Bullishly repeating his themes at the National Press Club in Washington, he pointed to the fact that many popular preachers talk as if US policy and Christianity were the same thing.

"When you start confusing God and government, you're in serious trouble." Amen to that. Accused of being unpatriotic, the former US Marine asked: "How many years did Cheney serve?" Answer: the gung-ho American Vice-President avoided the draft.

The Rev Jeremiah Wright is a complex and uncomfortable man, but he is no buffoon. It seems that it is impossible to speak the truth - especially in dramatic and robust forms - at an election time.

That coruscating truth has wounded Barack Obama, in a country in which a politician can be damned because of his pastor's rhetoric. Wright shrugs off the political impact by saying that politicians think about electability, while pastors answer to a higher authority.

This dualism isn't really good enough. Wright needs to use his intelligence and sophistication to discern when to speak out and when to hold his tongue.

Despite the inflammatory language he has a lot of truth on his side, but he also has to accept responsibility for collateral political damage in the here and now.

The irony is that as Democrats prepare to vote in North Carolina and Indiana tomorrow, a black preacher's passionate and politically naive rhetoric may yet keep America's great black hope out of the White House.