The chief rabbi of Rome received a special invitation from Pope Francis to attend a special mass on March 19.VINNews article. Normally, a Jew is not even allowed to visit a church, much less a religious service. But the Pope is a head of state, and the leader of the world's largest Christian movement, raising the issue of kavod hamalkus. Also, the rabbi would be going in an official capacity to promote good Jewish-Catholic relations. Do those facts give the rabbi a basis for going? What halachic theory can he rely upon?
Rome's chief rabbi, Rabbi Riccardo Di Segni, will attend Pope Francis' installation mass. But his attendance will not be without precedence: Rabbi Di Segni, who took the chief rabbi position in 2001, attended the funeral mass for Pope John Paul II. He wasn't the only Orthodox Jew there. Oded Wiener, director general of the Chief Rabbinate of Israel, represented the Chief Rabbinate at the funeral. Rabbi Shear-Yishuv Cohen, Chief Rabbi of Haifa, also attended. Rabbi Di Segni was invited to attend the 2005 installation mass for Pope Benedict XVI, but declined the invitation only because the mass was during Pesach.
The sources I cited do not give an explanation as to how Rabbi Di Segni, or any Orthodox Jew, can attend any Catholic mass, whether it is for a papal burial or the installation of a new Pope. But the Chief Rabbinate's decision to send their director general to John Paul II's funeral mass, would appear to say that there is indeed an exception to Rabbi Metzger's strong position on Jews entering a church (Rabbi Metzger was the Ashkenazic Chief Rabbi at the time). The exception is likely because the Pope is (a) a head of State, and (b) highly influential in Jewish-Christian relations around the world -- therefore maintaining favorable relations with him is essential for the safety to the Jewish people and Israel.
Some insight into Rabbi Di Segni's rationale may be gained by examining Rabbi Haskel Lookstein's rationale for attending a service honoring President Obama's first inauguration at Washington's National Cathedral, an Episcopalian church. Rabbi Lookstein gave the following reasons:
Chief Rabbi of Israel Rabbi Yona Metzger answered a shailah on entering a church in clear terms and gave sources for his decison.
Rabbi Riskin distinguished between entering churches not containing icons or statues (certainly permissible) and those that do (permissible for study of art or to know how to respond to non-Jews in discussions). Participating in a service, he writes, is forbidden except to learn as above or to prevent a desecration of God’s name (eg Chief Rabbi Sacks and Prince William’s wedding).
Rabbi Riskin’s opinion and some responses to it can be seen here.