Wednesday is the 150th anniversary of one of the most familiar and most misunderstood doctrines in Catholicism. It is the feast of the Immaculate Conception, which celebrates the belief that Mary, the mother of Christ, was conceived without original sin.

For centuries, it had been such a popular belief that the laity lobbied popes to make it an article of faith. The U.S. bishops dedicated this nation to the Immaculate Conception in 1846. In 1854, it was proclaimed dogma, one of two times in post-medieval history that a pope has claimed to have made an infallible declaration.

But today, the meaning of that dogma is often lost in confusion over who was being conceived and how. It's commonly misconstrued as the Virgin Birth, the belief that Jesus was born to a virgin. And TV comedians have joked they knew it was a bad omen when they were married in a Church of the Immaculate Conception. But the dogma is about neither sexless conception nor the birth of Jesus.

The Virgin Birth, the New Testament claim that Mary became pregnant with God's son through a miracle of the Holy Spirit, is upheld by all major Christian traditions. But the Immaculate Conception is unique to Catholicism. It holds that Mary was conceived free from the inherited guilt of the first sin committed by Adam and Eve.

"It's very easy to garble this with the virginal conception of Jesus," said the Rev. David Poecking, who has organized three seminars on the topic Tuesday at St. Paul Seminary in East Carnegie.

Mary's conception of Jesus is marked on March 25, nine months before Christmas. But "confusion is compounded by the fact that the church tends to use the virginal conception reading on the Immaculate Conception holy day," Poecking said.

That is because the angel's greeting to Mary in Luke 1:28 is considered an important biblical support for the Immaculate Conception. Some Bibles and a beloved Catholic prayer translate the greeting as, "Hail Mary, full of grace."

The phrase Catholics know as "full of grace" is the subject of great theological and linguistic debate. Many newer Bibles translate it as "highly favored."

However it is translated, the Catholic church understands it to mean that Mary was not affected by the original sin in Eden that doomed the human race to rebel against God. That, the church teaches, is why she alone was worthy and willing to bear God's son.

In parts of post-Reformation Europe, the idea of Mary's immaculate conception was as passionately debated as sports and politics are today, said the Rev. Thomas Thompson, who directs the University of Dayton's Marian Library, a repository of research and art about Mary.

Some centuries-old Spanish and Mexican churches bear inscriptions such as, "Don't let anyone enter this place without affirming with your life that Mary was conceived without the original stain."

That "original stain" baffles people today. Contemporary Americans can barely conceive of sin as something they do, much less an inherited condition. "We used to have a very strong notion that we were born in sin," Thompson said. "I don't know of anyone who is emphasizing that in the catechetical materials we have today."

As a result, Thompson has tried to find contemporary language to express the same idea, speaking of Mary's "sinlessness."

But because human behavior is so flawed, original sin is the only doctrine "for which we have social science evidence," said The Rev. Thomas Reese, editor of the Jesuit magazine America. The biblical story of Adam and Eve rebelling by eating forbidden fruit, he said, is supposed to explain that "the consequences of sin last generations; they are not just for those who committed the sin."Both Protestants and Eastern Orthodox reject the Immaculate Conception. They argue that all human beings, including Mary, are affected by original sin and needed Jesus to save them.

The Catholic church's answer is that Mary was saved from sin in an even more powerful way than anyone else. The Immaculate Conception movement reached a critical moment in 1830, when a young French nun, the future St. Catherine Laboure, had a vision of Mary standing on a globe with rays of light emanating from her hands. Around her image were the words, "O Mary, conceived without sin, pray for us who have recourse to thee."

In her vision, Catherine was told to strike a medal from the image, and that those who wore it would receive protection from the Mother of God. By 1900, more than 100 million had been distributed. The "miraculous medals," wildly popular in times of war and plague, still are in use today. In 1849, Pope Pius IX sent letters to all bishops to gauge the depth and breadth of devotion to the Immaculate Conception and ask whether their people and priests wanted him to declare it dogma. The bishops favored the idea 546-56.

On Dec. 8, 1854, Pius issued a letter defining the dogma, a doctrine that Catholics must accept. It was the first time a pope had defined a dogma without convening all the bishops, said the Rev. Francis Sullivan, professor of theology at Boston College.As such, it paved the way for the dogma of papal infallibility, which the bishops declared 14 years later.

Contrary to popular belief, most pronouncements popes make do not claim to be infallible. Only once since 1854 has a pope exercised his power to infallibly define a new dogma -- the 1950 declaration that Mary was assumed, or miraculously lifted, into heaven. Like the Immaculate Conception, the feast of the Assumption, celebrated Aug. 15, is a Holy Day of Obligation, meaning Catholics are required to attend Mass.

On Wednesday, Bishop Donald W. Wuerl will celebrate a special 8 a.m. Mass at St. Paul Cathedral, Oakland, and Immaculate Conception parish in Washington will have a 150th anniversary Mass and reception at 7 p.m.

The Tuesday seminars at St. Paul Seminary in East Carnegie will explore the implications of the Immaculate Conception and are aimed at people with some theological training. The workshops cost $10 each or $15 for both. For more information, call 412-921-5800, ext. 22.