John Paul II has been acclaimed as one of the most influential leaders of the twentieth century. It is widely held that he was instrumental in ending Communism in his native Poland and eventually all of Europe as well as significantly improving the Catholic Church's relations with Judaism, Islam, the Eastern Orthodox Church, and the Anglican Communion. Though criticised for his opposition to contraception and the ordination of women, as well as his support for the Second Vatican Council and its reform of the Liturgy, he has also been praised for his firm, orthodox Catholic stances in these areas.

He was one of the most-traveled world leaders in history, visiting 129 countries during his pontificate. He spoke the following languages: Italian, French, German, English, Spanish, Portuguese, Ukrainian, Russian, Croatian, Esperanto, Ancient Greek and Latin as well as his native Polish. As part of his special emphasis on the universal call to holiness, he beatified 1,340 people and canonised 483 Saints, more than the combined tally of his predecessors during the last five centuries. On 19 December 2009, John Paul II was proclaimed "Venerable" by Pope Benedict XVI.

Karol Józef Wojtyła (Anglicised: Charles Joseph Wojtyla) was born in the Polish town of Wadowice and was the youngest of three children of Karol Wojtyła, an ethnic Pole, and Emilia Kaczorowska, who was of Lithuanian & Polish ancestry. His mother died on 13 April 1929, when he was eight years old. Karol's elder sister, Olga, had died in infancy before his birth, thus he grew close to his brother Edmund, who was 14 years his senior, and whom he nicknamed Mundek. However, Edmund's work as a physician led to his death from scarlet fever, profoundly affecting Karol.

As a youth, Wojtyła was an athlete and often played football as a goalkeeper. His formative years were influenced by numerous contacts with the vibrant and prospering Jewish community of Wadowice. School football games were often organised between teams of Jews and Catholics, and Wojtyła would voluntarily offer himself as a substitute goalkeeper on the Jewish side if they were short of players.

In mid-1938, Karol Wojtyła and his father left Wadowice and moved to Kraków, where he enrolled at the Jagiellonian University. While studying such topics as philology and various languages at the University, he worked as a volunteer librarian and was required to participate in compulsory military training in the Academic Legion, but he refused to fire a weapon. He also performed with various theatrical groups and worked as a playwright. During this time, his talent for language blossomed and he learned as many as 12 foreign languages, nine of which he later used extensively as Pope.

In 1939, Nazi German occupation forces closed the Jagiellonian University after the invasion of Poland. All able-bodied males were required to work, and, from 1940 to 1944, Wojtyła variously worked as a messenger for a restaurant, a manual labourer in a limestone quarry and for the Solvay chemical factory to avoid being deported to Germany. His father, a non-commissioned officer army officer, died of a heart attack in 1941, leaving Karol the sole surviving member of his immediate family. “I was not at my mother's death, I was not at my brother's death, I was not at my father's death,” he said, reflecting on these times of his life, nearly forty years later, “At twenty, I had already lost all the people I loved.”

He later stated that he began thinking seriously about the priesthood after his father's death, and that his vocation gradually became ‘an inner fact of unquestionable and absolute clarity.’ In October 1942, increasingly aware of his calling to the priesthood, he knocked on the door of the Archbishop's Palace in Kraków, and declared that he wanted to study for the priesthood. Soon after, he began courses in the clandestine underground seminary run by the Archbishop of Kraków, Adam Stefan Cardinal Sapieha.

On 29 February 1944, Wojtyła was knocked down by a German truck. Unexpectedly, the German Wehrmacht officers tended to him and sent him to a hospital. He spent two weeks there recovering from a severe concussion and a shoulder injury. This accident and his survival seemed to Wojtyła a confirmation of his priestly vocation. On 6 August 1944, ‘Black Sunday’, the Gestapo rounded up young men in Kraków to avoid an uprising similar to the previous uprising in Warsaw. Wojtyła escaped by hiding in the basement of his uncle's home at 10 Tyniecka Street, while German troops searched upstairs. More than eight thousand men and boys were taken into custody that day, but he escaped to the Archbishop's Palace, where he remained in hiding until after the Germans left.

On the night of 17 January 1945, the Germans fled the city, and the students reclaimed the ruined seminary. Wojtyła and another seminarian volunteered for the unenviable task of clearing away piles of frozen excrement from the lavatories. That month, Wojtyła personally aided a 14-year-old Jewish refugee girl named Edith Zierer who had run away from a Nazi labour camp in Częstochowa. After her collapse on a railway platform, Wojtyła carried her to a train and accompanied her safely to Kraków. Zierer credits Wojtyła with saving her life that day. B'nai B'rith and other authorities have said that Wojtyla helped protect many other Polish Jews from the Nazis.

On completion of his studies at the seminary in Kraków, Karol Wojtyła was ordained as a priest on All Saints' Day, 1 November 1946, by the Archbishop of Kraków, Cardinal Sapieha. He was then sent to study theology in Rome, at the Pontifical International Athenaeum Angelicum, where he earned a licentiate and later a doctorate in sacred theology. This doctorate, the first of two, was based on the Latin dissertation The Doctrine of Faith According to Saint John of the Cross.