The now Father Karol said his first mass in Wawel Cathedral's crypt chapel. He stood among the sarcophagi of Polish monarchs and heroes. Very quickly after his ordination Father Karol was sent to Rome to study. He studied at the Pontifical International Angelicum (The future Pontifical University of Saint Thomas Aquinas) beginning in November of 1946, residing at the Belgian Pontifical College. Father Karol earned his a license in July of 1947 and passed his doctoral exam in 1948. His doctoral thesis was named Doctrina de fide apud S. Ionnem a Cruce (The Doctrine of Faith in Saint John of the Cross).
Alfons Stickler (the future Austrian Cardinal and school mate of Father Karol) wrote that Father Karol visited Saint Padre Pio during this time. Saint Padre Pio told him that he would
Ascend to the highest post in the Church
Father Karol returned to Poland in 1948 and was assigned to be the parish priest in the village of Niegowic at the Church of the Assumption. His first action was to kneel and kiss the ground to bless it, as he arrived at harvest time. Just one year later he was reassigned to the parish of Saint Florian in Krakow and taught ethics at Jagiellonian University. It was also during this time that he gathered a group of friends to discuss poetry, discussion on philosophy and helping those in need. The group began to call themselves the Rodzinka (Little Family) and would often go on kayaking trips in the mountains together.
He would also often take students with him on these retreats and trips to the mountains but asked the students to call him Uncle (Wujek) so that the villagers would not know that he was a priest. Not only were outdoor Masses not allowed by the Soviets but Priests travelling with groups of students was also strictly forbidden. This nickname (Uncle) would stay with him well into his Papacy - famously when he was named the auxiliary bishop many of his students and Rodzinka members were afraid he would shed this nickname and the attitude that came with it. He famously responded with
"Wujek will remain Wujek".
It was on these trips that Father Karol and the group would celebrate Mass in the open. In the new communist Poland, unapproved worship outside of churches was strictly forbidden. Father Karol would also publish several works of poetry and plays but under different names - often Andrez Jawien and Stanislaw Andrzej Gruda. He did this for two reasons - two have his works stand upon their own merits and to separate these works from his religious writings.
Just a few years later in 1954, Father Karol earned a Doctorate in Sacred Theology with a dissertation titled "Reevaluation of the possibility of founding a Catholic etic on the ethical system of Max Scheler". Unfortunately, due to the Communist Regime Father Karol was unable to receive this degree until 1957 because the Faculty of Theology at Jagellonian University had been abolished.
Impressed with his ability to operate such a strong and dynamic pastoral life in the face of severe Communist restrictions, Pope Pius XII named him an auxiliary bishop of Krakow.
He served as Auxiliary Bishop to Archbishop Eugeniusz Baziak and received episcopal consecration on September 28th, 1958. At the age of 38, Bishop Karol became the youngest ever bishop in Poland.
When Archbishop Baziak passed away in June 1962, Bishop Karol was selected as the Vicar Capitular (or temporary administrator) until a new Archbishop could be appointed.
Bishop Karol was invited to participate in the Vatican II council in October of 1962 and played a major role. The Decree on Religious Freedom (Dignitatis humanae) and Pastoral Constitution on the Church in the Modern World received heavy contributions. According to John F Crosby, Saint John Paul II used the words of Gaudium et spes to later introduce his own views on the nature of the human in relation to God:
Man is the only creature on earth that God has wanted for it's own sake, but man can fully discover his true self only in a sincere giving of himself
In January of 1964, impressed by the contributions in the council, Bishop Karol was formally appointed as the Archbishop of Krakow. Just three years later, on June 26th, 1967, Archbishop Karol was promoted to the Sacred College of Cardinals as the Cardinal-Priest of the titulus of San Cesareo in Palatio. He played an integral and instrumental role in the formation of the encyclical Humanae Vitae. During 1974 and 1975, Cardinal Karol served Pope Paul VI as a consultor to the Pontifical Council for the Laity.