Pope Benedict, insisting the faithful hold firm to church teaching, has told Roman Catholic politicians they must support the Vatican's non-negotiable rejection of abortion and same-sex marriage.

On Tuesday, Benedict also rebuffed calls to let divorced Roman Catholics who remarry receive Communion.

Putting his conservative stamp on his nearly two-year-old papacy, Benedict in a document also said the Vatican is keeping its requirement that priests be celibate, despite shortages of clergy in some parts of the world.

A worldwide meeting of bishops, held at the Vatican in 2005, endorsed the celibacy requirement and Benedict, with the document, embraced their call.

The 131-page "exhortation,'' destined for both clergy and rank-and-file faithful among the world's 1.1 billion Roman Catholics, is part of Benedict's vigorous campaign to ensure bishops, priests and their congregations strictly follow church teaching.

Before becoming pontiff in April 2005, Benedict, a German theologian, led the Vatican's drive to safeguard church teaching from doctrinal error.

Laced throughout the document are what sounds like nostalgic calls by Benedict for a kind of comeback for Latin and more "sobriety'' during mass.

Russell Shaw, a conservative Roman Catholic writer in the United States, described Tuesday's document as "certainly consistent with the pattern of this pontiff to date, a highly intelligent, highly thoughtful document which says nothing surprising but which reaffirms the traditional positions of the church.''

The question of whether Roman Catholic politicians whose politics conflict with church teaching should be denied Communion grabbed attention during the 2004 U.S. presidential election campaign, when St. Louis Archbishop Raymond Burke said he would deny the Eucharist to Democratic presidential nominee John Kerry, a Roman Catholic who supported abortion rights.

Benedict wrote that public witness to one's faith is especially required of politicians who decide matters such as abortion, euthanasia, "the family built upon marriage between a man and a woman...and the promotion of the common good in all its values.''

"These values are not negotiable. Consequently Catholic politicians and legislators, conscious of their grave responsibility before society, must feel particularly bound, on the basis of a properly formed conscience, to introduce and support laws inspired by values grounded in human nature,'' Benedict wrote.

Benedict indicated he is leaving the matter of wayward Roman Catholic politicians to local bishops.

"Bishops are bound to reaffirm constantly these values as part of their responsibility to the flock entrusted to them,'' the Pope said.

Referring to Benedict's leaving the matter to bishops, Lisa Sowle Cahill, a theologian at Boston College, said liberals might be "grateful he's not more aggressively insisting that pastoral flexibility be curtailed.''

The plight of divorced Roman Catholics who remarry is a concern for many faithful in the United States, where divorce and remarriage is common among the general population.

While Benedict acknowledged "the painful situations'' of those remarried Roman Catholics, he also reiterated the church's stance that they cannot receive Communion because the church views such faithful as living in sin if they remarry and consummate their new marriages.

The church "encourages these members of the faithful to commit themselves to living their relationship ... as friends, as brother and sister,'' Benedict said.

Benedict sounded rueful about some of the changes in the mass since the liberalizing reforms in the 1960s after the Second Vatican Council, including a switch from mass in Latin to local languages.

The Pope wrote he agrees with bishops at the 2005 meeting that, on international occasions, parts of the mass should be celebrated in Latin.

Faithful can be taught to recite the more common prayers in Latin, Benedict wrote.

Lately, on Wednesdays when the Pope meets with thousands of pilgrims and tourists, the faithful have been invited to recite the Lord's Prayer in Latin, with the text printed in the audience's program.

The 1960s reforms also inspired some congregations to replace sombre hymns with feet-tapping folk music and, in the name of peace, to exchange kisses or energetic hugs during mass.

"Certainly, as far as the liturgy is concerned, we cannot say that one song is as good as another,'' wrote Benedict, who plays classical music on the piano.

He suggested the "Gregorian chant be suitably esteemed and employed'' at mass and gestures of peace could be "restricted to one's immediate neighbours'' in the pews.