Five high-ranking cardinals have taken one of Pope Francis' favorite theologians to task over an issue dear to the pope's heart and critical to the Vatican's upcoming two-year study on family issues.

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."

Kasper has become the lightning rod for debate over a speech he gave cardinals in February at Francis' request. In it, Kasper suggested that Catholics who divorce and remarry without an annulment might be allowed in limited cases to receive the Eucharist after a period of penance.

Conservatives have vehemently opposed Kasper's proposals as contrary to Christ's teaching on the indissolubility of marriage.