Peter Mogila

In the 1620s, Mohyla traveled to Ukraine, which at that time was in political turmoil due to internal and external factors, in part due to Poland's annexation of Ukrainian lands. He started preparing spiritually at his aristocratic home in Rubezhivka near Kyiv, where he also founded a church dedicated to Saint John the New from Suceava. He then settled in the Kyiv Pechersk Lavra (the Monastery of the Caves), which was the political, cultural, spiritual, and educational center of Ukraine. There he joined Job Boretsky, Zacharias Kopystensky, and Pamvo Berynda, and a group of scholars and Orthodox clerics who promoted ideas of national liberation and cultural self-preservation. The prevailing political instability affected all spheres of life in the country. The number of printed publications was significantly reduced and many schools were closed. In order to preserve their privileges before the Polish king, the nobility, in great numbers, started to convert from Orthodoxy to Greek and Roman Catholicism.

The Romanians from Moldavia, Wallachia and Transylvania belonged to the Eastern Orthodox Church, and Old Church Slavonic was used, until the 17th century, as the common liturgical language in Romanian principalities.

In 1632 Mohyla became the bishop of Kyiv and abbot of the Kyiv Pechersk Lavra. Because of his ties to several European royal houses, the leadership of the Orthodox clergy entrusted him to negotiate with the Polish Sejm (parliament) and the king to lift the repressive laws against the Orthodox Church and to ease the restrictions on the use of the Church Slavonic language in schools and public offices. Mohyla's diplomatic talent paid off. King Władysław IV reinstated the status of the Eastern Orthodox Church in the Polish–Lithuanian Commonwealth. During his first years as abbot, Mohyla showed that he had far-reaching goals for reform not limited to the monastic life at the Lavra and the Church. He wanted to strengthen the Orthodox spirituality and enhance the sense of national identity as well as raise the educational level in the country and in all Ruthenian and Romanian lands to equal that in Western Europe.

One of Mohyla's first steps in implementing this vision was the founding at Lavra of a school for young monks in 1632. The tutoring was conducted in Latin. The students studied theology, philosophy, rhetoric, and classical authors. At the same time, Mohyla significantly improved the print shop at the Lavra where Orthodox books were published not only in Old Slavic but in Latin as well and distributed to various places in eastern Europe. Later that year, Mohyla merged this school with the Kyiv Brotherhood school and created the Mohyla Collegium, which later became known as the Kyiv Mohyla Academy (National University of Kyiv-Mohyla Academy). The students at the collegium had diverse backgrounds. They came from noble, clerical, gentry, Cossack, and peasant families. The school offered instruction in a variety of disciplines and languages: Church Slavonic, Latin, Greek, and Polish; philosophy; mathematics including geometry; astronomy, music, and history. Because of the high profile of the faculty, the collegium received the status of a higher educational establishment.

In the next few years, Mohyla established a whole network of schools around Ukraine as well as the Slavonic-Greek-Latin Academy in Vinnytsia and collegium in Kremenets. Also, he supplied the prince of Wallachia, Matei Basarab, upon his request, with a printing press and printers. In 1635 the prayer books which were published in Prince Basarab's monastic residence were widely distributed in Wallachia (later to become Romania) and Ukraine. He helped establish the school in Iași in Moldova as well.

Petro Mohyla was born into the House of Movilești, who were a family of Romanian boyars. Several rulers of Moldavia and Wallachia were members of this family, including Mohyla's father, Simion Movilă, thus making him a prince. He was also a descendant of Stephen the Great through the bloodline of his great-grandfather Petru Rareș. His paternal uncles were Gheorghe Movilă, the Metropolitan of Moldavia, and Ieremia Movilă, who also ruled Moldavia before and after the first reign of Simion. Petro Mohyla's mother, Marghita (Margareta), was the daughter of a Moldavian logothete, Gavrilaș Hâra. Petro Mohyla's sister Regina married Prince Michał Wiśniowiecki, and their son Jeremi Wiśniowiecki was Mohyla's supporter even though he converted to Catholicism in order to marry a Roman Catholic princess and thus inherit the Polish crown.

After Mohyla's father was murdered in 1607, Mohyla and his mother sought refuge in the Ruthenian Voivodeship, part of Lesser Poland. For a time, they lived in Kamianets-Podilskyi, and in 1608 they moved to Stanisław Żółkiewski's castle, where they stayed for sixteen years.