Pope John Paul II, beginning his pilgrimage through Poland's holiest shrines, reached out today for the spirit of millions of other Christians in Communist East Europe and for a younger generation of Poles whose connection to the church, he said, must be preserved.

At an emotional reception by tens of thousands of people at the ancient cathedral here, the pope talked about a delicate yet crucial theme for the church - the fate of millions of Slavic Christians and Russian Orthodox believers in neighboring Communist lands, especially Czechoslovakia and the Soviet Union.

In a departure from his prepared text, the pope called attention to a banner he had seen in front of the church held up by a group of visiting Czech Catholics. "Remember, Father, your Czech children," the banner read. The pope said it was good that he had seen this and he would not forget them.

Other Eastern European countries are less tolerant of the church than Poland, and in many places border areas are thought to be closed to prevent people from crossing into Poland to see the Pope.

In another departure, he drew attention to the need for freedom of speech - restricted in many Communist-ruled nations - by saying "it would be sad to believe that each Pope and Slav in any part of the world is unable to hear the words of the pope, this Slav." He added: "I hope they hear me . . . We cannot forget these brothers of ours."

The pope went on to add that "we are living in a time of declared freedom and exchange of information."

Earlier in a dusty meadow just outside this town that, a thousand years ago, was the first capital of Polish Catholicism, a colorful sea of people - unofficially estimated at up to half a million - waited for five hours in 90 degree heat to see and listen to their most famous native son.

The crowds came from all over, from Gniezno and surrounding villages, from the larger city of Posnan just to the west, and from cities as far away as Gdansk on the Baltic Sea coast.

Many said they had walked long distances. A number, including several nuns, collapsed from heat exhaustion in the sun and crush of people.

When the pope arrived by helicopter from Warsaw, crowds rushed to the barriers to meet him and he mingled easily, shaking hands and moving steadily toward the huge white altar that had been set up in the meadow.

"He's home, he's home," shouted one woman, and thousands greeted him with Polish chants wishing him a long life.

The crowds here were mostly working people and farmers. There were many old and middle-aged people.

In Warsaw yesterday, on the first day of his nine-day trip through his native country, the pope had touched on sensitive political as well as religious themes - stressing fundamental human rights and the indelible link between Catholicism and Poland's history, even though it now exists within an officially atheistic communist world.

In the meadow outside Gniezno today the pope's theme was simpler, more religious. He was here, he said, "in the nest . . the cradle" of both the church and the motherland.

Mostly, however, the pope stressed that a fundamental role of the church and its members is to teach the young because only they can ensure the faith of future generations. In effect, the pope asked parents and teachers to keep children within the church as a means of preserving the faith not only within an imposed Communist ideology but also at a time when Poland, like many other countries, is faced with some movement away from formal religion by young persons.

"May all the children of preschool age have easy access to Christ," he said, and he warned against ignoring Jesus' words, "Let the children come to me."

Despite the enormous crowds, there was no hysteria or tumult in the field. As in Warsaw the day before, the throng responded with dignity and respect along with the warmth that has clearly enveloped the pope during the first days of his return to Poland.

One senses in these crowds an obviously deep well of carefully restrained devotion to the alternate, spiritual leadership embodied by the church in this country of 35 million persons, about 90 percent of whom are Catholics.

Shortly after dawn, before coming here, the pope held a mass for students and young people that drew another huge crowd into the winding, cobblestoned streets of Warsaw's old town. He spoke with special warmth to the youth of Poland.

Many of long-haired and casually dressed students had spent the night on the sidewalk outside St. Anne's Church - known as the "church of the young" - to catch a glimpse of the pope on his way to say mass.

As the crowd sang the hymn, "Christ is Our Shield, "tears appeared in the pope's eyes. Then he was crying openly.

"If only you knew how much I love you, my little ones," he said in a choked voice.

Then John Paul was driven to Vicotry Square where on Saturday he had said mass for a quarter of a million people. The vast open area was bare except for two gleaming white helicopters.

Crowds gathered on the periphery cheered when the pope arrived and as the helicopters left, taking him and his entourage to Gniezno.

Driving into Gniezno from the meadow, the pope enjoyed what was perhaps the most spirited and colorful welcome yet, surpassing Warsaw in vigor in not in size.

Tens of thousands of people lined the streets and crammed windows and balconies, waving flowers and Polish and Vatican flags, singing and chanting in a display of warmth.

At the cathedral, the pope said, in effect, that neither the church nor he personally could fail to take notice of the problems of Christians in other Communist societies, even though their language and plight may not be as familiar to churchgoers elsewhere in the world.

He wondered aloud to the congregation, "is it not Christ's will . . . that this Polish pope, this Slav pope, should at this precise moment manifest the spiritual unity of Christian Europe?"

"We cannot fail to hear also . . . other Slavic languages" as well as our own, the pope said. And, "these languages cannot fail to be heard especially by the first Slav pope. Perhaps this is why Christ has chosen him . . . in order that he might introduce into the communion of the words and the languages that still sound strange to the ear accustomed to Romantic, Germanic, English and Celtic tongues."

Is it not time, he asked, for the church to turn with "understanding and special sensitivity," to these other forms of speech.

Monday the pope - the first ever to visit a Communist country - continues what has turned out to be a demanding schedule in the intense and surprising spring heat. The pope will visit Czestochowa in Southern Poland, site of the Jasna Gora sanctuary and home of the famed Black Madonna Byzantine icon. CAPTION: Picture 1, Pope John Paul II greets a throng of believers in a meadow outside Gniezno, Poland, on the second day of his visit to his homeland. AP; Picture 2, Pope waves to crowds surrounding Warsaw's Victory Square before boarding Helicopter to fly to Gniezno. UPI; Map, no caption, By Richard Furno - The Washington Post