VATICAN CITY – The battle lines are being drawn before a major church meeting on family issues that represents a key test for Pope Francis.

Five high-ranking cardinals have taken one of Francis’ favorite theologians to task over an issue dear to the pope’s heart: Whether Catholics who divorce and remarry without an annulment can receive Communion.

They have written a book, “Remaining in the Truth of Christ,” to rebut German Cardinal Walter Kasper, whom Francis praised in his first Sunday blessing after he was elected pope as “a great theologian” and subsequently entrusted with the setting the tone for the two-year study on marriage, divorce, and family life that the Vatican will open Oct. 5.

Kasper, for a decade the Vatican’s top official dealing with the Orthodox and Jews, delivered an introductory speech to cardinals earlier this year on the issues to be discussed during the synod in which he asked whether these divorced and remarried Catholics might be allowed in limited cases to receive the Eucharist after a period of penance.

The outcry that ensued has turned the 81-year-old Kasper into the biggest lightning rod for internal debate that the Catholic Church has seen in some time.

Conservatives, including the five cardinal authors, have vehemently opposed Kasper’s suggestion as contrary to Christ’s teaching on the indissolubility of marriage.

The second most powerful man in the Vatican has backed their view: Cardinal George Pell, one of Francis’ key advisers, wrote in another new book that debating something that is so peripheral to begin with and so clear in church teaching amounts to “a counterproductive and futile search for short-term consolations.”

“Every opponent of Christianity wants the church to capitulate on this issue,” he wrote. “We should speak clearly, because the sooner the wounded, the lukewarm and the outsiders realize that substantial doctrinal and pastoral changes are impossible, the more the hostile disappointment (which must follow the reassertion of doctrine) will be anticipated and dissipated.”

Francis, however, seems to think otherwise. He praised Kasper’s speech and confessed that he even re-read it before going to bed the night after Kasper delivered it. He called it “profound theology” that did him much good and represented a true love for the church.

The debate – unusually raw and public for such “princes of the church” – has crystalized the growing discomfort among conservatives to some of Francis’ words and deeds, and sets the stage for what is likely to be a heated discussion on family issues for the foreseeable future.

In a Vatican news conference today, Italian Cardinal Gianfranco Ravasi said that the debate over the indissolubility of marriage is nothing new, since it’s been around in the Church since its origins.

A Bible scholar by training, Ravasi noted that the Gospel of Matthew includes an exception to the general ban on divorce for porneia, a Greek word that’s usually translated to mean “fornication,” but which has been read over the centuries to mean a variety of different things.

“We don’t know exactly what it was, but we know that the primitive Christian community recognized an exception to the ban on divorce, without questioning Jesus’ message over the indissolubility of the sacrament,” he said.

During the Oct. 5-19 Synod of Bishops on the family, Ravasi will preside over a commission to produce the meeting’s final message. When asked today about the divorce debate, he initially refused to answer, insisting that the press conference was intended to promote an upcoming event he’s staging in Bologna.

In the end, however, Ravasi couldn’t resist, and his reference to the early exception on divorce seemed to align him with those bishops at least open to the case for softening the communion ban.

Church teaching holds that Catholics who don’t have their first marriage annulled – or declared null by a church tribunal – before remarrying can’t participate fully in the church’s sacraments because they are essentially living in sin and committing adultery. Such annulments are often impossible to get or can take years to process, leaving untold numbers of Catholics unable to receive Communion.

Francis has asserted church doctrine on the matter but has called for a more merciful, pastoral approach: He reportedly told an Argentine woman earlier this year that she was free to receive Communion even though her husband’s first marriage was never annulled. Knowing the issue is divisive, though, he has convened the whole church to debate the issue.

The new book asserts there really is no better solution – and no grounds to argue for it citing the practice of the ancient church – since Catholic doctrine is clear. The authors include two of the highest-ranking Vatican officials, both appointed by Emeritus Pope Benedict XVI, a great friend to conservatives and traditionalists: Cardinal Gerhard Mueller, head of the Vatican’s doctrine office, and Cardinal Raymond Burke, the American head of the Vatican’s supreme court.

“These are not a series of rules made up by the church; they constitute divine law, and the church cannot change them,” the book says. Kasper’s assertions, reading of history and suggestions for debate “reinforce misleading understandings of both fidelity and mercy.”

Kasper has agreed there can be no change to church doctrine and no sweeping, across-the-board allowances. But he has said that the matter must be looked at on a case-by-case basis, that mercy is God’s greatest attribute and the key to Christian existence, and that God always gives faithful Catholics a new chance if they repent.

It is rare for cardinals to publicly and pointedly accuse another cardinal of being flat-out wrong, and rarer still for a cardinal to question the pope, as Burke has done. Regarding the purported phone call to the Argentine woman, Burke told the EWTN Catholic channel: “I wouldn’t for a moment impute that Pope Francis intended to give a signal about church doctrine by calling someone on the phone. This is just absurd.”

Burke has also questioned Francis’ first encyclical on the excesses of capitalism and obliquely criticized Francis’ decision to not focus on abortion.

Francis last year removed Burke, a key figure in the U.S. culture wars over abortion and gay marriage, from the advisory board of the powerful Congregation for Bishops. A leading Vatican insider has reported that his days at the Vatican high court are numbered, even though he is well below the retirement age.

The books are published in English by Ignatius Press, which handled Benedict’s English-language publications.