Pope Benedict XVI is under fire again, after issuing a new document reaffirming the traditional doctrine that the Catholic Church is the one true apostolic church — the only one instituted by Jesus Christ.

According to the doctrine, Protestant denominations are not strictly churches but “ecclesial communities” because they lack valid priestly orders and sacraments.

The document recognizes that Orthodox Christians belong to “sister churches,” as they have preserved the apostolic succession of the priesthood. But their refusal to recognize papal authority means these churches suffer from “defects.”

Titled “One Church,” the document was issued by William Cardinal Levada, whom the pope appointed as his successor to lead the Congregation for the Doctrine of the Faith.

Its language will inevitably offend many non-Catholic Christians, few of whom will read it but who will take its exclusive-sounding theology as an implied challenge. Episcopalians may see the document as a hostile response to the ordination of women and gays.

Yet the document is not primarily addressed to other churches but to heterodox Catholics, whose “erroneous” interpretation of the ecumenical ideas of the Second Vatican Council 40 years ago have led them to ignore the scandal of schism, and even to treat all churches as equally valid.

In fact, “One Church” says nothing new. It follows closely the line laid down by “Dominus Jesus,” an earlier document issued in 2000 by the then Cardinal Ratzinger.

“One Church” starts from the premise that Jesus founded only one church, not a plurality. Other Christians share elements of “sanctification and truth” with the Catholic Church, and to that extent also share its universal mission.

“These separated churches and communities are deprived neither of significance nor importance in the mystery of salvation. … In fact, the spirit of Christ has not refrained from using them as instruments of salvation,” the document states.

This recalls the “Nostra Aetate” document of Vatican II, which modified the old formula “no salvation outside the church” to give all Christians, Jews, and other faiths their proper place in God’s plan for humanity.

“One Church” is not hostile to closer ties with other denominations, but it reminds Catholics that the fruits of ecumenical dialogue come “from that fullness of grace and of truth which has been entrusted to the Catholic Church.”

Some critics see this new document as part of a “traditionalist” counterrevolution that includes the ending of restrictions on the use of the old Tridentine Mass, which they claim signals a reversion to old-style Catholic anti-Judaism.

The pope’s admirers inside and outside the Catholic Church may worry that he has once again unnecessarily polarized opinion. Others will admire his forthright stand, intended to shore up the foundations of a faith that is under both internal and external attack.

It is important to recall that this pope made a point of inviting the Eastern churches that are in communion with Rome to take a prominent part in his inaugural Mass in 2005. He sees it as his mission to continue Pope John Paul II’s highly successful attempt to reach out, especially to the Orthodox churches and to the Jewish people.

In his recently published book, “Jesus of Nazareth,” Benedict emphasized the “Jewish Jesus,” while on his visit to Turkey he made special efforts to reconcile with the Greek Orthodox patriarch of Constantinople.

Pope Benedict wants to bring the Catholic Church into close alliance with other Christians, including Protestants, and also with Jews, who are similarly threatened by the rise of radical Islam.

Benedict XVI is known to be deeply concerned the persecution of Christians, especially in the Middle East and other Muslim states.

By affirming the unity and uniqueness of the Catholic Church, he is sending a message to the tens of millions who are suffering for their faith.

He is also carrying out his promise to resist radical secularism, especially in Europe. To that end, he wants to eliminate any doctrinal ambiguities that may “give rise to confusion and doubt.” The Catholic Church that Benedict hopes to leave behind him will have a clearer sense not only of what it teaches but of what it is.