Speaking to a rally of more than 100,000 people, Archbishop Christodoulos sharply escalated a fight that strikes at some bedrock issues: the essence of the Greek identity and whether political leaders can truly govern without the consent of the Orthodox hierarchy.
``Your efforts are futile,'' cried Christodoulos in an obvious attack against the long-governing Socialists. ``The people do not follow you ... You think you will accomplish you plans with the sword of power. You are mistaken.''
``For our faith, we will be lions,'' he told a cheering crowd.
Rally organizers had predicted a turnout of 500,000, but the crowd appeared to fall well short of their goal.
The showdown began with government plans to strip the religious affiliation entry from state ID cards.
The government says privacy laws require the change, which is strongly supported by rights groups and religious minorities such as Jews and Muslims who claim the IDs make them targets for discrimination.
The church and many followers see the religious entry as an important symbol of the historical role of the Orthodox church, which sees itself as the true guardian of Greek identity.
But larger--and more volatile--issues lurk just below.
For church leaders, there is the fear of possible moves toward a formal separation of church and state in Greece. About 97 percent of the native-born population are baptized into the Greek Orthodox Church.
The government is faced with another major hurdle in the nation's difficult transition from its traditional order to a more Western model based on muting ethnic and religious differences.
Evoking the names of Greek independence heroes, Christodoulos portrayed the church as the genuine voice of the nation and warned the government of the potential risks of challenging its wishes.
``Many before you have tried and failed...All of the Greek people will resist,'' said Christodoulos, who is listed in polls as one of Greece's most popular and influential figures.
``We will remain what we are,'' he proclaimed, ``Greeks and Christians first, then Europeans.''
Such words shake the pillars of Premier Costas Simitis' government, which is preoccupied with trying to bring Greece into the mainstream of the European Union and shake its unruly image.
Government spokesman Dimitris Reppas complained the church was adopting a ``political character ... not in line with its role.''
Parliament speaker Apostolos Kaklamis called the rally ``a mob, not the people.'' "I fear the situation in the church could spawn great dangers for the future of this country,'' he said.
The peaceful, seaside rally took on the atmosphere of a pilgrimage as black-clad monks, zealots with crosses and others traveled from across northern Greece--by foot, train, in car convoys and aboard hundreds of chartered buses. Some came from as far away as Australia.
Peddlers sold Greek flags and banners adorned with the two-headed eagle symbol of the Byzantine Empire. Baseball hats carried the message: ``I am Greek Orthodox.'' Some children held balloons of Pokemon cartoon characters.
Church leaders are furious with Simitis for rejecting any dialogue on a possible compromise.
``The church had to take to the streets,'' said a theology student, Irineiou Verikakis.
Conservative lawmakers have encouraged the church. But general elections are not scheduled for nearly four years and the Socialists have a comfortable majority in parliament.
The crusade also has rocked the church itself. Some top clerics have refused to join the protests, saying the ID issue is purely a political issue.
In another blow, the international spiritual head of the Orthodox faith, Istanbul-based Patriarch Bartholomew I, issued a letter to Simitis saying the ID reforms are the ``jurisdiction of the state,'' according to the government-run Athens News Agency.
Police were on full alert around Thessaloniki. Church leaders have urged its followers to be peaceful, but the rally could also draw ultra-nationalists and other fringe groups.
Religious feelings are particularly strong in northern Greece. The Muslim Ottomans still held land there less than 100 years ago and the affinity is deep for the Greek-dominated Byzantine Empire that ended with the fall of nearby Constantinople, now Istanbul, in the 15th century.
Another rally is planned for Athens on June 21.