Pope John Paul II, cracking down on what he considers serious abuses in his flock, issued a stern reminder Thursday that divorced Roman Catholics who remarry cannot receive communion.
The pontiff also warned Roman Catholics against taking communion in non-Roman Catholic churches drawing immediate criticism from some Protestant leaders for what they call a step backward in efforts to achieve Christian unity.
John Paul's warnings were contained in an encyclical, a special letter reserved for matters of extreme importance to the church.
"It is my hope that the present encyclical letter will effectively help to banish the dark clouds of unacceptable doctrine and practice," the pope wrote, going on to denounce abuses and confusion over "sound faith and Catholic doctrine" concerning communion.
Although the pope has made better relations with other Christians a cherished goal of his papacy, his alarm that some Roman Catholics may be diluting their faith by pursuing those efforts irritated other Christians.
Domenico Tomassetto, a member of the ecumenical relations commission of the Baptist, Waldensian and Methodist churches in Italy, said the encyclical made "a definite cut to the entire ecumenical process." Protestants are a tiny minority in Italy.
John Paul's crackdown regarding communion, while breaking no new ground, also was likely to stir up anger in the United States and other countries where divorced and remarried Catholics have been hoping that someday the pope might reverse the ban on their taking communion.
Vatican teaching forbids divorce and considers Catholics who remarry after divorce to be living in sin.
Although the pope did not specifically cite the plight of divorced Catholics, experts said his intent was clear.
"That is what the Vatican is saying for years about the non-admission of public sinners to the Eucharist whether they be divorced or Mafia or people who are notorious criminals," said the Rev. Thomas Reese, editor of America, a Jesuit magazine.
The pope, who consistently has been conservative on doctrinal matters, said Roman Catholics must examine their conscience to determine whether they have sinned, but church law holds that those who "obstinately persist in manifest grave sin" be denied communion.
He wrote that "in cases of outward conduct, which is seriously, clearly and steadfastly contrary to the moral norm, the Church, in her pastoral concern for the good order of the community and out of respect for the sacrament, cannot fail to feel directly involved."
John Paul also sought to tighten up practices in some communities where he sees the line blurring between strict Church teaching and popular practices.
He reminded faithful that only priests can celebrate Mass, even in parishes short of priests.
The pope also branded as "unthinkable" the practice of substituting obligatory Sunday Mass with celebrations of prayer with other Christians or participation in their liturgical services.
Roman Catholics, "while respecting the religious convictions of these separated brethren, must refrain from receiving the communion distributed in their celebrations," he said.
Roman Catholics maintain that they receive the body and blood of Christ in communion, but many other Christians view communion as a symbolic re-creation of the Last Supper.
Germany's Lutheran church said the pope's "strict" interpretations underscored the rift among Christians. Berlin will host a major ecumenical conference next month.
"It must soberly be recognized that a common view about ... communion does not appear possible at this time," said Christof Vetter, a Lutheran church spokesman.
German Cardinal Karl Lehmann, who heads the German Bishops Conference, acknowledged there were "great obstacles" to closer Christian unity.
"The pope knows of the pain that Christians feel about this. But he pleads urgently not to let hopes for the unity of the church dim because of this," the cardinal said.
Archbishop of Canterbury Rowan Williams, the Anglican leader, welcomed what he called the pope's reaffirmation of his "burning" desire for common Eucharistic celebration, adding "we continue to work theologically on this."
The pontiff did say there were some special circumstances in which other Christians could take communion from a Roman Catholic priest to meet the need for "eternal salvation." Likewise, Roman Catholics could ask ministers from other faiths with "valid" sacraments to give them communion, confession or final rites.