CZESTOCHOWA, Poland — Pope Francis, visiting a shrine cherished by Poles, praised native son St. John Paul II on Thursday as a "meek and powerful" herald of mercy, and honored the countless "ordinary yet remarkable people" in Poland who held firm to their Catholic faith throughout adversity in the former Communist nation.

The Argentine pontiff, who had never had set foot in Eastern Europe before this week's five-day pilgrimage, gazed in awe for several minutes as he studied the Jasna Gora monastery shrine's iconic image of the so-called Black Madonna and Child. The faces in the images are blackened by centuries of varnish and candle soot since the artwork became the object of veneration, starting in the 14th century.

A silver screen was raised slowly and dramatically to unveil the image as Francis stood silently in admiration and then prayed.

Then, during an outdoor Mass near the southern town of Czestochowa, before a crowd that appeared to number more than 100,000, Francis lavished praise on Poland's steadfast Catholic legacy as he urged Poles to hold fast to their faith.

Pope Francis missed a step and fell to the ground as he was coming to an open-air altar to celebrate Mass. In a dramatic moment, Francis, walking deep in thought in his long robe, did not notice a step down and fell to the ground before the altar. Priests around him rushed to help him up and straightened his robe.

The Mass proceeded as planned and the pope delivered a long sermon before tens of thousands of faithful gathered at the foot of the Jasna Gora monastery in the southern city of Czestochowa.

The Mass was held in celebration of the 1,050th anniversary this year of the Poland's acceptance of Roman Catholicism. The baptism of a medieval king in 966 put the nation on course to be part of the Latin-speaking Western world, setting it apart culturally from Orthodox nations in the region.

"Our minds turn to so many sons and daughters of your own people, like the martyrs who made the defenseless power of the Gospel shine forth, like those ordinary yet remarkable people who bore witness to the Lord's love amid great trials," Francis said in his homily, speaking in Italian and pausing for a priest to translate his words into Polish.

He then cited two beloved Polish saints, praising those meek and powerful heralds of mercy who were Sts. John Paul II and Faustina. "Through these channels of his love, the Lord has granted priceless gifts to the whole Church and to all mankind."

Francis then recalled the far less famous soldiers of the faith in Poland.

"Your own history, shaped by the Gospel, the cross and fidelity to the church, has seen the contagious power of a genuine faith, passed down from family to family, from fathers to sons and above all from mothers and grandmothers, whom we need so much to thank," he said.

Francis then urged today's Poles to stay united, as their nation is divided over such issues as how to view refugees and migrants, especially those who aren't Christians. He prayed that Poles would have "the desire to leave behind all past wrongs and wounds, and to build fellowship for all, without ever yielding to the temptation to withdraw or to domineer."

The pope did not elaborate. A right-wing government came to power in Poland last year that has faced international scrutiny for eroding past democratic gains, and many critics say they see a rise in xenophobic sentiment in the country.

Worry about bad weather prompted a last-minute change in his day's travel plans, with the pontiff opting to take a car instead of a military helicopter to Czestochowa, 100 kilometers (60 miles) from Krakow. But the gray skies held into the Mass.

Francis' visit to Poland centers around a Catholic youth jamboree that has drawn hundreds of thousands of participants this week. He will have his first big meeting with the young faithful in a Krakow meadow Thursday evening.

En route, Francis made an unscheduled stop at a clinic to visit and pray for comatose Cardinal Franciszek Macharski, an-89-year-old retired prelate who had been archbishop of Krakow. Marcharski had replaced Cardinal Karol Wojtyla in the post after the latter was elected the world's first Polish pontiff, John Paul II, in 1978.

With John Paul a national hero as well as a beloved saint, Francis on this five-day trip finds himself in a deeply Catholic country that is attached to Czestochowa, where the shrine is located, and where a main boulevard is named after John Paul.

In his first speech after arriving Wednesday, Francis called on Poles to be compassionate and welcoming to those in need, especially refugees. He also met with Polish President Andrzej Duda, whose populist government has virtually slammed the door on asylum-seekers and economic migrants from the Middle East and Africa who are flooding southern European shores. The president also attended the Mass at the Jasna Gora shrine.