Despite the difficulties, John issued a new statute of Vilnius cathedral chapter in 1520, called the first known diocesan synod in 1520 or 1521, and, with pope's permission, created two new prelates of the cathedral chapter in 1522. John also received papal legate Zacharias Ferrerius sent to investigate canonization of Saint Casimir after a miracle attributed to him during the Siege of Polotsk. John paid attention to education. In 1522, he revised the curriculum of the Cathedral School of Vilnius to include rhetoric, dialectics, classical literature, arithmetic, music. In 1526 or 1527, John called the second diocesan synod which debated three major topics: improper behavior by priests, proper procedures of church services, and establishment of schools. The results of the proceedings were published in Kraków in 1528. The synod decided that each parish church should have a school that would instruct children in both Polish and Lithuanian languages. The synod also forbade hiring German teachers or traveling priests as they could be influenced by Lutheranism. His father Sigismund I the Old gifted John with Šiauliai in 1524, Zhytomyr in 1525, and Kremenets in 1529. That was enough to make him a very wealthy magnate – the military census of 1528 placed him the ninth wealthiest. He funded repairs and reconstruction of Vilnius Cathedral and its belfry. In June 1530, a large fire devastated Vilnius Cathedral. The repairs were slow and Italian architects Bernardino Zanobi de Gianotis and Giovanni Cini were hired only in July 1534. John sponsored construction of a new church in Šiauliai in 1526. He established new parishes and built churches in Joniškis, Gervyaty, and Zarasai. In total, during his tenure as Bishop of Vilnius, more than 30 new churches were built in the diocese. On 18 October 1529, John crowned his half-brother Sigismund II Augustus as Grand Duke of Lithuania. In June 1535, John was appointed overseer of Vilnius Mint when it temporarily reopened during the Muscovite–Lithuanian War. In March 1536, Queen Bona Sforza, who disliked John, managed to get John removed from Vilnius to the Diocese of Poznań. He died there just two years later but, according to his last will, he was buried in Vilnius Cathedral. A new chapel, known as the Chapel of Bishops or Chapel of the Holy Sacrament, was built according to his last will around 1548. His tomb monument of Pińczów limestone was ordered by his brother Sigismund Augustus in 1556. It was designed by Giovanni Maria Mosca also known as Padovano, but did not survive.