While the exact date is unknown, Lucaris was ordained in Constantinople. In 1596 Lucaris was sent to the Polish–Lithuanian Commonwealth by Meletius Pegas, Patriarch of Alexandria, to lead the Orthodox opposition to the Union of Brest-Litovsk, which proposed a union of Kiev with Rome. For six years Lucaris served as professor of the Orthodox academy in Vilnius (now in Lithuania). In 1601, Lucaris was installed as the Patriarch of Alexandria at the age of twenty-nine. He would continue to hold this office for twenty years, until his elevation to the See of Constantinople. During these years, Lucaris adopted a theology which was heavily influenced by Protestant Reformation doctrine. On 6 September, he wrote a letter to Mark Antonio de Dominis, a former Roman Catholic Archbishop, writing: