The unceasing struggle between the Red and the Black enters a new phase with the papacy of John Paul II. As a Pole the pope automatically disengages the Vatican from Italian politics in a way that hurts the local Communist party.

As former bishop of Krakow he embodies and will intensify a policy which has already loosened the grip of communism in Eastern Europe. Hence his call to "open up frontiers" in his inangural mass on Sunday.

The critical distinction between the new pope and even his most recent predecessors lies in the matter of church and state. Mixing the two came naturally to the Italian popes, the more so as a Catholic party, the Christian Democrats, ruled the roost in Italy since the end of World War II. Only four years ago Paul VI threw all his prestige and that of the Vatican into a long, drawn-out, losing campaign against legalizing divorce in Italy.

But John Paul II hails from a country where the government was more enemy than friend to the church. Far from wanting to mix the two, he shares with the bishops of northern Europe, who helped make him pope, an instinct for holding church and state apart. As he said in his first message as pope: We have no intention of political interference, nor of participation in the working out of temporal affairs."

The most obvious beneficiary of that approach in Italian politics is the Christian Democratic Party. Though once helped by the support of local church organizations, the Christian Democrats have been increasingly burdened by the anachronistic views of the Vatican on such social issues as divorce and contraception.

As a result, the Christian Democrat's share of the Italian vote has sunk steadily from about half in 1948 to just over a third in 1976. Under Prime Minister Giulio Andreotti, the Christian Democrats now govern as a minority dependent upon support from the Communists. Short of some inner transformation, the Christian Democrats cannot avoid eventually teaming up with the Communists as coalition partners in the so-called "historic compromise."

But the papacy of John Paul II provides the opening for an internal change. With support from the Vatican withdrawn, the Christian Democrats now have no good alternative but renovation. They have to adopt more modern social policies, and broaden their appeal beyond the peasants and the Catholic middle class. As the Italian editor Eugenio Scalfari wrote the other day, "Now more than ever, they must become a lay party."

In Eastern Europe, the papacy of John paul II sets up an opposite effect. For about a dozen years now, the church has followed an Eastern policy, or Ostpolitik , of give and take. While yielding on such issues as exoneration of Cardinal Mindszenty, the church has been insisting on its right to preach, teach and exercise moral sway. If the results have been only partly successful in East Germany, Hungary and Czechoalvakia, in Poland the policy has been triumphant. Not only has church attendance risen, and the number of seminary students soared but also, the chruch has become an unofficial opposition to which the government repeatedly turns for approval.

Extising ties between the churches of Eastern and Western Europe, moreover, were knitted much more tightly together. Thus alongside Eurocommunism, there has developed a Eurocatholicism.

With the accession of Pope John Paul II, the spread of Western values through the church to Eastern Europe takes on even greater force. The new pope himself is talking about visiting not only Poland but also Russia. His ascent means that Moscow - faced with a revitalized American president, a tougher Japan, a more active China and a Near East on the road to settlement - has also to cope with a Western Europe capable of renewing itself.