''The Tiber has become wider,'' Romans say about the river that divides Vatican City on the right bank from the very secular capital of Italy on the left.
In this Roman Catholic heartland, the Tiber has not become so wide as to introduce the comprehensive separation of church and state envisaged in Protestant democracies. But this year's signing of a new concordat - an agreement that for the first time in more than half a century no longer accords Catholicism the status of the sole official religion of Italy - is indicative of a historic loosening of ties between church and state throughout Catholic Europe.
In the new concordat the Catholic Church has consented to the loss of its official status in Italy. Catholic religious instruction in the schools now is an elective rather than a compulsory subject. This means parents must now take the initiative to sign their children up for religious instruction rather than asking to have them exempted as in the past. Church annulments of marriage are now subject to state approval (and state insistence on a father's financial support for children).
The state may also phase out its payments for parish priests and church buildings by 1990. Last year the government paid $175 million for priests' salaries and $12 million for construction of new churches.
The concordat represents one more step in the evolution of church-state ties that has been accelerating dramatically in Italy and Spain over the past decade. Even France, which started the whole tremor two centuries ago, is still experiencing aftershocks. A result of 18th century liberalism
In a way today's developments are the logical consequence of the liberal, anticlerical revolt that began with the French Revolution and swept all of Europe in the last century. Depending on one's point of view, the present adjustments in Italy, Spain, and France represent either the inevitable and continuing secularization which resulted from the liberal revolt or else a reconciliation between the Roman Catholic Church and liberal political forces after more than a century of conflict.
Italy probably offers the clearest example of the trends, as befits the land that gave birth to the Catholic Church and is still the center of the worldwide Roman Catholic hierarchy. The 109-acre Vatican City is not itself Italian in that it is recognized diplomatically - now even by the United States - as a sovereign state. But the Curia, or Vatican court, has been overwhelmingly Italian for virtually all of its long centuries of history. And the indivisibility of Vatican and Italian politics was taken for granted until 1870 - and again, in a different way, after 1929 and even after 1945.
What occurred in 1870 was the unification of Italy under vehemently anticlerical northern Italians. Symbolically, the last step of unification was the march into the Vatican's Rome by Italian troops. Vatican territories were annexed, and the Holy See, or papal court, was reduced to its present enclave. The rest of Rome became the capital of the new Italy. By 1917 Italy's ruler, Victor Emmanuel, would be the only king in the world to be a declared atheist.
The aversion was mutual. In the Syllabus of Errors of 1864, Pope Pius IX branded democracy one of the world's main evils (along with Protestantism, Freemasonry, communism, socialism, parliamentarianism, and freedom of worship). And when Italy was unified for the first time in 1,100 years, Pope Pius declared that none of his successors would ever accept the Holy See's loss of temporal power. He retreated to the Vatican for life, excommunicated the royal house, and forbade Roman Catholics to run for office or to vote in Italian elections. The ban lasted until 1904.
The liberals failed in their attempt to govern the new state. Even after decades of formal political unity neither the Italian public nor the fraction of the population with the franchise managed to transform their intense localism into national patriotism. Seventy percent of the populace was illiterate, and the only element of the Enlightenment that had an impact on the governing class in Italy was anticlericalism - which fueled the drive for national unity. All this meant there was no preparation of Italians for individual participation in democratic politics. And the long boycott of Italian politics by many Catholics in a population that was 99 percent Catholic didn't help. Reinstated as state religion
The large majority of Italians therefore welcomed it when Benito Mussolini took over as dictator and instituted a Fascist regime in 1922. And they welcomed it as well when Mussolini made peace with the Vatican in the concordat and other Lateran pacts of 1929.
In these treaties Catholicism again became the official state religion on the Italian peninsula. In arrangements that would carry over into postwar Italy, Catholicism was again taught in the public schools by priests - the only religion so taught. The state paid for all religion instructors in the public schools and also subsidized alternative private schools run by Catholics. However, the state did not accede to the church's demand that the state inspect the entire public school curriculum to ensure its compatibility with Catholic teaching.
After World War II Italy reestablished itself as a democratic state. In religious affairs the 1947 Constitution guaranteed freedom of worship. But, ambivalently, Article 7 explicitly extended the validity of the Lateran treaties as well, and Article 36 identified Catholicism as the only official religion.
In the postwar readjustments it became clear the Catholic Church no longer considered participatory democracy the work of the devil, nor deemed monarchic or authoritarian rule the only God-given form of government. Pius XII became the first Pope to speak positively of parliamentary democracy - and went on to stress the importance of ''the unity of the Catholics in politics.'' The conservative Christian Democratic Party that had sprung up in the early 1900s was revived, with the blessing of the church, and politicians, hierarchy, and public alike took it for granted that the Christian Democratic leadership would have intimate ties to the Vatican. At this point both party and church saw themselves as partners in a mortal struggle with the Communists to preserve democracy and religion in Italy.
The alliance enjoyed enormous success. Italians, insecure about their own fragile political institutions and awed by the centuries-old stability of the church, accorded the Vatican a deference liberal critics condemned as servility. The Christian Democrats dominated Italian politics for four decades (and continue to do so today).
Nor was Vatican intervention in politics confined to backroom deals. Until the late 1950s Italian bishops played an active role in selecting Christian Democratic candidates and lobbying for them. Apostolic letters were read in churches that made it quite clear how parishioners were expected to vote - and the church fully backed the ban on socialists as well as communists in public administration.
Even more deplorable, in the eyes of that devout Catholic and towering historian of modern Italian church-state relations, A. C. Jemolo, was the hier-archy's backing of the secret plots of some right-wing Christian Democrats and old Fascists to reinstate authoritarian government in the capital. The aim was to prevent Rome - which Pius XII insisted must retain its sacred character - from falling into the hands of socialists and communists. These plans were never carried out, but Catholics did manage to postpone implementation of constitutionally prescribed regional autonomy until the 1970s.
The results of the postwar Catholic dominance of politics were unfortunate for some of Italy's small number of Protestants. In particular, the Pentecostalists suffered real persecution, Professor Jemolo pointed out. And the indigenous Protestant Waldensians - whose history traces back to medieval heresies - again found themselves disadvantaged. It would take until the late 1950s for the Italian Supreme Court to end the persecution of Pentecostalists with rulings which favored religious liberty. Still, the discomfort of the minuscule numbers of Italian Protestants - and of Catholic intellectuals of democratic conscience like Professor Jemolo - was little more than a pinprick to the accepted assumptions of a natural bond between church and state. Even the reforming Pope John XXIII - despite his extraordinary endorsement of political pluralism - had no intention of revising the 1929 concordat and the Catholic Church's status as the officially recognized state religion.
What John XXIII did do on his accession to St. Peter's throne in 1958 was to deemphasize politics in favor of the church's pastoral mission - and to turn the church out of Italian isolation toward a new, worldwide role. On the centenary of Italian unity he expressed the novel papal view that the church's loss of temporal power was a good thing. It was the hand of God, he said, leading the church to a more universal vision.
John's successor, Paul VI, may have been more conservative in intramural church and ecumenical affairs, but he continued John's withdrawal of the Vatican from everyday Italian politics. Under what was called ''the religious choice,'' the church detached itself further from the active politics of the 1950s.
Moreover, by something of a fluke, Paul VI came to reconsider the 1929 concordat. What happened, according to one of the best-informed Italian chroniclers of the contemporary Vatican, Benny Lai, was that a performance of Rolf Hochhuth's ''The Deputy'' was scheduled in Rome in 1965.
The play, a sensation at the time, accused Pope Pius XII of having turned a blind eye to and indirectly aided the extermination of Jews in Hitler's Germany. The Vatican, invoking 1929 residual rights under which Rome was still a ''holy city,'' got the play banned.
This political censorship aroused unease. Paul VI began to talk with his friend and then-Christian Democratic prime minister, Aldo Moro, about revising what suddenly seemed to be an anachronistic concordat left over from the very different era of Mussolini. Formal church-state negotiations on revision started in 1967.
Over the next 17 years of on-again-off-again negotiations, a whole new generation of Italians grew up. Industrialization and urbanization resulted in far-reaching sociological shifts - and transformed the entire context of the revision. The barometers of change were the public referendums on divorce in 1974 during the papacy of Paul VI and on abortion in 1981 under the present Pope , John Paul II. Church influence on politicians eroded
In an unprecedented move in 1970 the Chamber of Deputies had passed legislation permitting civil divorce as distinct from church annulment.
This first parliamentary flouting of an explicit pronouncement by the Pope - on issues of marriage and divorce Paul VI would and did intervene in politics - revealed an erosion of church influence on politicians. But it was nothing compared with the 60-to-40 public rejection of church counsel in Italy's first referendum since the 1946 decision to become a republic. And even the divorce revolt was mild compared with the public's 70-to-30 approval of abortion.
Almost all observers - including the Vatican - were astounded.
The public attitudes expressed in the referendums - and John Paul II's own character - accelerated work on revision of the concordat. As the first non-Italian Pope in almost 500 years, John Paul II (the former Karol Cardinal Wojtyla) eschewed direct church intervention in Italian politics (on issues not involving personal morals). As a Pole, he may not have been pluralist by nature. But as a Roman journalist puts it, ''With Wojtyla, he detaches himself from Italian political life in a sense, because he only worries about Poland.''
Certainly with his Polish background of struggle with a hostile state for the very survival of the church, John Paul II also tends to regard some of the Italian issues as less than critical. The Polish church derives all its income from parishoners, and therefore a few lira more or less in the Italian state's underwriting of the Catholic Church seem immaterial to him. The spontaneous ardor of simple Catholics is what keeps worshippers crowding into the Polish churches, and therefore the designation or nondesignation of the Catholic Church as the official religion is irrelevant.
In addition, John Paul II is impatient with details. He finally instructed the negotiators to expedite settlement of the concordat revision, without haggling over the fine print, according to a veteran European journalist with access to Vatican officials. In February of this year the new document was signed, in basic outline, leaving the sorting out of such sticky questions as tax liability of church-owned property and businesses to the further definition of a church-state commission. Some bishops, Radical politicians demur
The Italian bishops' conference - which from now on will replace the Vatican as the interlocutor with the Italian state - has expressed some veiled reservations about the concordat. Some of the Italian bishops (though perhaps not the majority) regret forfeiture of the cachet of official religion and the enshrining of the neutrality of the state in religious matters. Many more have misgivings about making religious education voluntary and fear a sharp drop in enrollment - and in future church adherence.
Many wonder, too, if the new division of ecclesiastical entities into nontaxable, purely religious institutions and taxable commercial enterprises should not have been defined concretely before signature of the concordat, while the church had more bargaining power.
For quite opposite reasons some Protestants and the handful of legislators who voted against ratifying the concordat also take a dim view of the agreement. ''Traditionally the Radical Party has been in favor of separation of church and state,'' notes Radical deputy Massimo Theodori. He points out the remaining privileges of the Catholic Church in Italy, not only in religious education and state subsidies but also in financial dealings (as the Ambrosiano bank scandal demonstrated) and in the Vatican's stranglehold on much real estate in Rome.
''The concordat reinforces (those Catholics who are) more interested in worldly affairs and worldly power, and it weakens (those Catholics who are) in favor of basing the strength of the church and religion only on spiritual positions,'' says Theodori.
From a religious viewpoint the united Waldensians and Methodists also express reservations about the concordat. They appreciate their own intesa (understanding) with the state, signed immediately after the concordat (after 36 years of negotiations). They are glad that marriages conducted by their pastors are now legally valid, that Waldensian theological degrees are now recognized, and that their pastors now have guaranteed access to prisons, institutions of care, armed forces bases (and schools, if requested). But the Waldensian weekly La Luce editorialized last February:
''The new concordat does not sweep away the Catholic Church's privileged status vis-a-vis the state.... The Constantinian principle of integration of church and state authority remains intact: Under the new Concordat the state still underwrites the cost of Catholic ministrations ... and the state still authorizes one denomination - the Catholic Church - to teach religion in public school.''
The Waldensians, who eschew any financial support from the state, do not seek any equalization by acquiring the same privileges for themselves. But the Jews, who are next in line to sign an intesa, would like to get at least some comparable financial support from the state - and are therefore postponing signature until this issue is resolved.
So has the 1984 concordat finally ended the 115-year-old conflict between clerical and anticlerical militants in Italy?
''Yes,'' says the negotiator of the concordat and the intesa in the prime minister's office, Prof. Gennaro Acquaviva, ''I think this conflict has basically come to an end. But this conflict actually didn't exist any longer in the consciousness of the Italian people. Deep in society there was no more conflict. The only conflict came over the privileges the church still had going beyond what is normal.''
And the assessment of an Italian Protestant: The Catholics ''have lost the battle, no doubt.... The Vatican is no longer Italy. But if you don't reckon with this phenomenon (the Catholic Church), you are not in a position to understand Italy.''