"It's not," theology professor Margaret O'Gara says, "like a hotline to God."
Rather, infallibility is the belief that God will not allow his church to veer too far on central questions of faith.
"It's not that everything the church teaches will be exactly right, but the gospel will not be completely lost," says O'Gara, who teaches at University of St. Michael's College in the University of Toronto and often writes about infallibility. Her 1988 book, Triumph in Defeat, explores papal infallibility.
James F. Keating, who teaches theology at Providence College in Rhode Island, puts it this way: "It's a charism, a gift given to the entire church that it will be preserved in the truth."
And it makes sense, Keating says, that every faith has its own notion of infallibility, though that word is not used much outside Catholicism.
Jews, Muslims and non-Catholic Christians have their own ways of believing that God protects believers from error.
"In a way, you have to believe it," Keating says. "You have to believe that what you believe has been protected by God from human manipulation."
For most Protestants, he says, that belief revolves around the inerrancy of the Bible.
The Catholic take on that, explains Stephen Ray, a former Baptist turned Catholic apologist, is that the gift of infallibility guided the early Catholic Church to decide which Christian writings belonged in the Bible in the first place.
"I like to say, 'When Jesus ascended to heaven, he didn't yell back down, "Be sure to read my book," ' " says Ray, who wrote the 1999 book Upon This Rock.
In the beginning • Indeed, most of the infallible teachings of Catholicism the dogmas were defined in Christianity's earliest centuries, O'Gara says.
For instance, early ecumenical councils defined central tenets such as Christ's divinity and humanity, the Trinity and the necessity of grace for salvation, she says.
The creeds written by early Christians spell out the most central dogmas.
In the past 200 years, there have been two instances in which a pope has defined a teaching as infallible in a solemn "ex cathedra" declaration, which is one of three ways the charism of infallibility is employed.
The first was in 1854, when Pope Pius IX declared Mary's Immaculate Conception, the belief that the mother of Jesus was born without sin.
The second also pertained to Mary. In 1950, Pope Pius XII deemed that Mary was assumed into heaven at the end of her earthly life.
The other two ways the Catholic Church determines beliefs to be infallible are through the bishops in concert with the pope either in a worldwide ecumenical council or not.
Most church dogmas have been defined by ecumenical councils, which occur, on average, once every century. The other way is more theoretical, which is why some confusion exists over the infallibility of certain teachings, such as that on contraception.
By that method, a teaching can be considered infallible if the bishops, in concert with the pope, say that it is a belief of the church. Most theologians say that's not the case with birth control, or, indeed, any moral issue; some traditionalists insist that it is.
There was no dogma defined at the most recent ecumenical council Vatican II in the 1960s. In fact, Keating says, Pope John XXIII explicitly said the world needed pastoral care, not more dogma, when he called the council.
The last time an ecumenical council handed down an infallible teaching was at the first Vatican Council in 1870, when the bishops approved the dogma of papal infallibility, meaning the pope can, at times, declare teachings on faith or morals to be infallible even without consulting the other bishops.
When the church declares a dogma, it's not that the belief is new, Ray says. It's that the church is challenged by rival interpretations and has to define what scripture and tradition teach on the matter.
Letters by first-century Christians, for instance, indicate that, from the start, the church believed the bishop of Rome the pope possesses the authority to sort out true belief, he says.
Moreover, not every core Christian belief has been declared infallible. Some, such as Christ's resurrection, have never been seriously challenged, O'Gara says, and are assumed to be central.
While the notion of infallibility is "very, very ancient," the word was not used until medieval times, she says.
The charism of infallibility is understood as protecting the church from error, not guaranteeing the church knows the full truth, Keating says.
"Basically," Keating says, "the argument is, no matter how evil a pope was and there were a lot who were evil none led the church into doctrinal error."
Consequently, many dogmatic definitions explicitly reject error, but do not expound on the truth because the church can teach only what it knows from scripture or sacred tradition.
In that way, Keating adds, the church acknowledges that not everything has been revealed.
"Whenever human beings speak about divine things," he says, "we're often as wrong as we're right."
Levels of assent • So how are Catholics to regard church teachings whether infallibly taught or not?
Canon law spells out varying levels of assent required of Catholics, O'Gara says. Those teachings considered infallible the dogma are not optional. Catholics are required to give unconditional agreement.
But other teachings, such as those in letters from bishops or encyclicals from popes, require only conditional assent, O'Gara says.
"You say, 'I submit myself to this teaching. I take it seriously … but finally it could be wrong and could be rejected,' " she says.
"You give it the strong benefit of the doubt … [because] you expect to hear the truth from the church in these matters."
There are instances of the pope and bishops changing course on moral teachings, she notes.
The church for centuries condoned torture for the sake of the truth, but it was condemned by the bishops meeting at Vatican II.
Pope John Paul II dissented from a long tradition of support for capital punishment and, toward the end of his life, was almost a pacifist in his examination of the doctrine of a just war in the modern world.
One of the most common misconceptions about infallibility among Catholics, O'Gara says, is the assumption that whatever a pope teaches soon will be dogma.
"People think that's what they're supposed to believe, and they leave the church," O'Gara says. "That isn't Catholic teaching, though."