Evangelical Protestants and Roman Catholics -- the nation's two largest religious blocs -- have a relationship that's been marked in the past by hostility and tension.
But now, almost 500 years after Martin Luther, one of the top U.S. evangelical thinkers has co-authored a book that finds an increasingly warm relationship between Catholics and evangelicals.
''Is the Reformation Over? An Evangelical Assessment of Contemporary Roman Catholicism'' (Baker) by Mark A. Noll and Carolyn Nystrom argues that not only on contemporary political issues such as abortion but also on matters of spirituality Catholics and the Protestant conservatives have ever more in common.
Summarizing the situation, Noll -- a historian at Wheaton College in Illinois -- said in an interview that he sees ''quite serious differences, but not differences of life and death as they were regarded for at least four centuries.''
He believes three events in particular fostered harmony:
--Catholicism's Second Vatican Council (1962-65), which encouraged contacts with Protestants and abolished for good the papacy's one-time hostility toward democracy and freedom of conscience in religion.
--The U.S. Supreme Court's 1973 liberal ruling on abortion, which provoked joint social activism, now reinforced by the same-sex marriage issue.
--The 1978 election of Pope John Paul II, who became a hero to evangelicals for helping topple European Communism and for speaking effectively on behalf of Christian tradition.
Other religious thinkers beyond Noll and Nystrom, a freelance writer who attends Noll's Presbyterian church, agree that the climate is warming.
''The admiration for John Paul II is simply astounding given (evangelicals') historic real hatred for the papacy,'' says William Shea of the College of the Holy Cross.
His 2004 work ''The Lion and the Lamb: Evangelicals and Catholics in America'' (Oxford) parallels the Noll-Nystrom book from the Catholic side. If anything, he thinks, Pope Benedict XVI is closer to the evangelicals' outlook than John Paul II.
Even Michael S. Horton of Westminster Seminary California, an evangelical who remains sharply critical of Catholic theology, says that ''the perceived cultural collapse of the West has become such an overwhelming preoccupation of conservative Catholics and Protestants that just about anything and everything else is on the back burner.''
The political turnabout since the 1960 presidential campaign was neatly framed last year by religious right activist Gary Bauer:
''When John F. Kennedy made his famous speech that the Vatican would not tell him what to do, evangelicals and Southern Baptists breathed a sign of relief. But today evangelicals and Southern Baptists are hoping that the Vatican will tell Catholic politicians what to do.''
Though the culture wars command much of the media attention, Noll and Nystrom are more interested in spiritual links between the two groups, which combined make up more than half of all American churchgoers. A notable example: When the Rev. Billy Graham first preached in Cracow, Poland, the man who invited him was in Rome being elected Pope John Paul.
Meanwhile, on the official level, the Vatican and the U.S. Catholic bishops are much more engaged with the centrally organized ''mainline'' Protestant churches than the conservative evangelicals.
That increases the importance of unofficial talks, in particular ''Evangelicals and Catholics Together,'' led by Catholic priest Richard John Neuhas of First Things magazine and prison evangelist Charles Colson. Participants have issued four joint statements (Noll endorsed the first two, while Horton was opposed) and are at work on a fifth.
Noll and Nystrom acknowledge that evangelicalism can be prickly and puzzling -- a complex movement consisting of denominations, local congregations, independent organizations and individualistic leaders.
Like Noll, Shea thinks most ongoing disagreements stem from two radically different views of the church.
''We Catholics are churchy people and we have a stack of beliefs about the church and perceptions of the church that evangelicals don't have,'' Shea says, calling this ''the big block that is almost insurmountable.''
Evangelicals famously champion the Reformation principle of ''Scripture alone'' as the source of religious authority, whereas Catholicism enshrines both Scripture and tradition as interpreted through the church. Other examples include the authority of the papacy and its dogmas about Mary.
Yet both Noll and Shea believe the evangelicals are much closer to Catholicism on central Christian teachings than more liberal Protestants.
So, does that mean the Reformation is over? Noll summarizes: ''The answer is not yes, but it's moving in the direction of yes.''