The Argentine pontiff, whose defence of the poor and vulnerable has stirred up the winds of change within the centuries-old institution, rocked traditionalists last weekend by marrying couples in Saint Peter's Basilica who had been living in "sin" according to the Church's rule book.
Of the 20 couples gathered in the heart of the Vatican to take their vows with the leader of the Roman Catholic church's blessing, some had already been cohabiting and one bride was a single mother.
In a mass in his private chapel, Francis also said the Church must "accompany... Not condemn" those whose marriages break down.
Out of the uproar among conservatives has come a book, "Remaining in the Truth of Christ", which brings together the opinions of five leading Catholics -- including Kasper's most vocal critic Gerhard Ludwig Muller, prefect of the doctrinal watchdog, the Congregation for the Doctrine of the Faith -- and four other experts.
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The book's editor, Robert Dodaro, sums up the essays in the introduction, saying the cardinals "are united in firmly maintaining that the New Testament shows us Christ forbidding without ambiguity divorce and remarriage."
"The 'merciful' answer to divorce endorsed by Cardinal Kasper is not unknown in the old Church but none of the authors here, which we consider to be authoritative, defend it. On the contrary, when they mention it, it is rather to condemn it as going against the scriptures," he writes.