Pope John Paul II, vowing to press efforts for Christian unity, arrived here today to a formal reception by his Turkish hosts and then virtually disappeared from public view behind a wall of extraordinarily tight security.

In the first day of a three-day visit in which he hopes to advance the reunification of the world's one billion believers of Roman Catholicism and Eastern Orthodoxy, the pope was almostcompletely isolated from a largely disinterested public.

He arrived at an almost deserted airport, was quickly spirited out of sight in an army helicopter and only once, during a brief visit to the mausoleum of Ataturk, was permitted to be driven through the streets of the capital of 2 million Moslems. Even then security forces cleared the streets of any traffic, and the papal motorcade went almost unnoticed past the handful of passerby.

While the first day here was devoted to talks with the leaders of the new government of Prime Minister Suleyman Demirel, the pontiff's main purpose in coming is his meeting Thursday with Dimitrius I, the spiritual leader of the Eastern Orthodox churches.

A high Vatican official, Bishop Ramon Torella Cascante, said the leaders of the Latin and Eastern rites, which drew apart more than nine centuries ago, would announce the formation of a joint commission to begin a formal dialogue on unity.

Before he left Rome to demonstrate the importance of the churches' relations "on the eve of the start of a theological dialogue," the pope said this was his first ecumenical trip and that he was "ready to go now" to London to meet with the Anglicans.

Even for a country that has been under martial law as long as Turkey has, the papal visit was shrouded in the tightest security in memory, because of purported assassination threats by rightist terrorists and the volatility of the Islamic fundamentalist movement here and in surrounding Moslem nations.

The pontiff, wearing a white cassock and scarlet cape, knelt and kissed the ground as he alighted, just as he had done when he arrived in the United States. However,, in striking contrast to his American visit, the airport was enveloped in almost disconcerting silence as the pope was greeted by Demirel, Turkish President Fahri Koruturk, opposition leader and former premier Bulent Ecevit and other Turkish leaders.

As he stood on a platform and listened to the Vatican and Turkish anthems, accompanied by a 21-gun salute, the pope smiled warmly toward the stony faces of a military honor guard. A momentary diversion from the meticulously planned but seemingly virtually mechanical welcome came when the brisk winter wind blew the pope's cape over his head and he smiled at the awkwardness of attempting to right it.

Shortly before landing, the pontiff, appearing in good spirits and obviously relishing his fifth trip outside Italy in a year, chatted with newsmen aboard his jetliner about the timing of his visit and the possibility of danger.

Asked if he felt there was danger in making the trip, the pope replied "Love is stronger than danger." He said the timing was simply designed to coincide with Friday's feast of St. Andrew, when Roman Catholic leaders traditionally visit the patriarch of eastern Orthodoxy in Istanbul.

When asked about ferment in the Islamic world, the pope said, "I esteem Islam as a monotheistic religion. That is true. My impressions were formed by the Vatican Council." While the Vatican Council offered no generic definition of Islam, it expressed respect for the monotheistic religions, and Vatican sources traveling with the pope interpreted his remarks as meaning he views Islam in that narrow religious context and not in the context of current political events occurring in Moslem nations in the Middle East.

After a visit to the residence of the Vatican's emissary in downtown Ankara and a courtesy call to the presidential palace, the pope went to the nearby presidential residence for a visit with Koruturk. In all of his stops, most of the press and all of the public was screened from the pope's view.

At the mausoleum, the pope laid a wreath and wrote, in French, an inscription in the guest book: "The government of the peoples is in the hands of God. He creates at the right moment the leader who suits them because love of liberty and respect of the law makes a nation great, but it is God who secures its future."

Tomorrow, the pope will fly to Istanbul, where he will meet with religious leaders of Eastern Orthodoxy led by the patriarch of Constantinople, Dimitrius I.

On Friday, He will visit Ephesus.