CINCINNATI, April 17 (AP) - During a lifetime cut off from organized religion, Aubrey ``Mac'' McCray had gone through three failed marriages and three desperate tries with Alcoholics Anonymous before he reached sobriety. An A.A. friend invited the retired auto worker to visit the suburban Vineyard Community Church, and there he reached a turning point.

McCray heard about a course just starting at Vineyard for those who know little about the faith or aren't sure what to believe. It's called Alpha and could be described as Christianity 101. An insider jokes it might just as well be called ``Agnostics Anonymous.''

At the tenth and final Alpha session, ``the tears just came rolling out like a flood broke loose,'' McCray recalls. ``I couldn't stop. It's just hard to describe.'' Alpha ``softened my heart,'' he says. ``It changed my life,'' he says.

McCray was one of 16 people who testified to Alpha's effectiveness last month when the Vineyard held a two-day conference promoting the program to U.S. clergy and lay leaders.

Housewife Sandy Ventura, who worships at the Covenant Fellowship in Glen Mills, Pennsylvania, says that before taking the classes she believed in God but, ``I didn't have a relationship with him.'' Now, she says, it is ``like a whole new life beginning for me.''

Most responses are less dramatic. But Alpha has undoubtedly cut a swath in the eight years since the concept spread beyond Holy Trinity Brompton church in London, which had quietly run Alpha courses since 1977.

Alpha has reached into 113 countries and 34 languages, according to Tricia Neill, executive director at the course's world headquarters located at the church.

Roughly 3 million people in many Christian denominations have now attended worldwide, including nearly 500,000 in the United States, where work began in 1996. Both those totals have doubled the past two years. Some 17,000 congregations participate.

No one is more astonished by this surge than the Rev. Nicky Gumbel, 45, Alpha's wiry leader and curate of Holy Trinity, located in the fashionable Knightsbridge section of the British capital.

``None of this was really planned. All the way along, it has been responding to a demand out there,'' he says.

Public interest, if not demand, should rise in Britain when a spiritual takeoff on the ``Survivor'' show, hosted by Sir David Frost, hits commercial television this summer. The series will track 10 Londoners week by week as they respond to Alpha sessions at Holy Trinity and reconsider Christianity.

In England, where worship attendance is low and getting lower, Alpha has ``revolutionized many churches and made many new Christians,'' Archbishop of Canterbury George Carey told a Billy Graham evangelism conference last August.

Similar enthusiasm is spreading among clergy in the United States, where the plan seems to fit all sorts of churches.

At the Cincinnati meeting, local Episcopal Bishop Herbert Thompson Jr. said, ``The Alpha program is a marvelous gift to the church throughout the world.''

Baltimore's Roman Catholic Cardinal William Keeler says he has heard ``wonderful testimony'' about results from Catholic sessions.

Top Evangelical Protestants - including Bill Bright, Tony Campolo, Charles Colson and Pat Robertson - have added their endorsements.

A noted layman, pollster George Gallup Jr., told a Philadelphia meeting that through Alpha, ``I have seen lives transformed, as mine has been.''

Less reverentially, the Financial Times calls Alpha ``a slick global conversion machine.''

For all the hallelujahs, Alpha uses a wholly unremarkable format. People are simply invited to meet weekly to enjoy a friendly free meal, listen to Gumbel, on video, or a live speaker chat about basic beliefs, then break into small discussion groups for a half hour.

Though everything is carefully designed to attract seekers and dropouts, Alpha presents an uncompromising conservative message.

It teaches that Christians shouldn't marry outside the faith, sleep together before marriage or dabble in the occult. And the manual guiding discussion leaders on typical questions has a chapter that says homosexual activity ``goes against God's created order.''

What church people think will appeal to outsiders is usually wrong, observes Holy Trinity's head priest, the Rev. J. A. K. ``Sandy'' Millar. He assigned Gumbel to run Alpha in 1990 and says his assistant reoriented the course toward people with no previous Christian involvement by reading thousands of questionnaires from Alpha attenders.

Gumbel was well suited to the task, as a thoroughgoing skeptic who converted to Christianity at Cambridge University after reading the New Testament straight through.

The Gumbel-era Alpha seeks to bring people's doubts to the surface rather than suppressing them. The first session is boldly titled, ``Christianity: Boring, Untrue and Irrelevant?''