Giese was the fifth child of Albrecht Giese and his wife, Elisabeth Langenbeck, both members of a wealthy merchant families. His paternal family had emigrated from Cologne to Danzig in the 1430s. His father was the Mayor of Danzig, and his mother's uncle, Johann Ferber, had been Mayor of Danzig. At the age of 12 years, Tiedemann, along with his cousin, Johann Ferber, entered the University of Leipzig, and subsequently studied at Basel and in Italy. He earned a Master of Theology degree. Giese was one of the best educated scholars in Prussia, well versed in both theology and the sciences. At age 24, he and Mauritius Ferber became priests at the Catholic Church of St. Peter and St. Paul. He was secretary to the King of Poland, and later appointed canon of Frauenberg, where he remained for 30 years. His residence was the Episcopal Castle at Frauenberg. The King appointed him Bishop of Kulm on 22 September, 1537. Toward the end of his life, he became Bishop of Ermland. Giese was supported by Chancellor Lucas David. He was a humanist and a liberal in the Erasmian mould. Although a Catholic, he demonstrated relative tolerance towards Lutherans. He made himself the spokesperson for a group of liberal and tolerant men who wanted to mediate between the "old-believers" and "the new-believers". In his writings, he expressed the aim of reconciling he Catholic and Protestant branches of the church, but ultimately alienated both of them. Bishop Giese was a lifelong friend and frequent companion of the astronomer and proponent of heliocentrismNicolaus Copernicus and shared his interest in astronomy. As a very wealthy man, Giese had the best instruments which, from time to time, he loaned to Copernicus. Giese, seven years younger than Copernicus, was sufficiently well educated to be able to follow Copernicus' studies. Giese bought his friend an ingenious sundial, and gave him an instrument with which he could observe the equinoxes. The mathematician, Rheticus, published a list of Giese's astronomical instruments, which he considered to have been made by men who really understood their mathematics. Giese actively encouraged his friend, Copernicus, to publish his findings in relation to the movement of the planets in the solar system. In turn, Copernicus regularly acknowledged his indedebtness to the many friends, especially Giese and Rheticus, who had supported him and encouraged him to publish. In 1516, Giese was the co-author, with Copernicus, of a letter to the Polish KingSigismund I the Old asking for the King's protection of Prussia against the Teutonic Knights, and generally supported the interests of the Polish Crown against that of the Teutonic Order. Danzig had been part of the Teutonic order since the Teutonic invasion of Gdańsk Pomerania in 1308 and although its population was ethnically German, it had achieved independence. The region's inhabitants resisted becoming part of the Poland, but turned to the Polish King for support against the Teutonic aggressors. He also worked on updating the Kulm law while a canon at Ermland. On 1 July 1536 he was designated by King of Poland, Sigismund I, who considered him a very valuable diplomat, as Bishop of Kulm, which was later confirmed by the Pope. After Mauritius Ferber's death Giese became prince-bishop. Giese was described, by his contemporaries, as a very pious man, an extremely learned man, a loyal friend, generous and a man who supported those in need. Nicolaus Copernicus explained that his "devoted friend, Tiedeman Giese, a man filled with the greatest zeal for the divine and the liberal arts." Giese was a prolific writer, and correspondent. He regularly corresponded with young, ambitious scholars, encouraging them and following their careers, with interest. In the preface of Copernicus book, Die Revolutionibus, he credits Giese with encouraging publication and urging him not to conceal the principles on which he deduced his theory of planetary motions. He carried out active correspondence with the humanist Erasmus of Rotterdam; the Lutheran reformer, Philip Melanchthon and the humanist philosopher, Damião de Gois. Among his known publications is Centum et decem assertiones, quas auctor earum Flosculos appellavit de homine interiore et exteriore and Antilogikon, a polemical refutation of Lutheran reformer Johann Briesmann. He also wrote De Regno Christi in which he envisions a reformed, reunited and reinvigorated church, but which now only survives in fragments. He also wrote a treatise defending Copernicuss' theory of planetary motions. Most of his other works have been lost, including a treatise on Aristotle. His friend, Copernicus willed his writings to Giese and left his library to the church administration of the Prince-Bishopric of Ermland. Bishop Giese died in Heilsberg and was laid to rest next to Copernicus at the Frauenburg cathedral.
