The pope and several Roman Catholic monarchs.
Pope Leo X, who was pope from March 9, 1513, to December 1, 1521, excommunicated Luther.
Martin Luther, professor of biblical studies and Augustine monk, initially sought to reform in the use of indulgences. In response, Prierias, Master of the Papal Palace, declared any challenge to the sale of indulgences heretical. Luther proceeded to deny the infallibility of the pope and of General Councils, for which the pope excommunicated him in 1520. However, the Elector of Saxony declined to enforce the Bull of Excommunication, which Luther publicly burnt in the university at Wittenberg. One of the forty one heresies and "pestiferous errors" of which he was accused was the opinion that "the burning of heretics is against the will of the Holy Spirit."
Martin Luther was declared an outlaw and heretic because of his criticisms of the Catholic Church and his teachings that challenged its authority. He argued against practices such as the selling of indulgences and questioned the supremacy of the Pope. These ideas gained a significant following and threatened the Catholic Church's power, leading to his excommunication and being labeled an outlaw by the Holy Roman Emperor, Charles V.
The Pope issued a papal bull threatening to excommunicate Luther if he did not recant in 90 days. Luther is said to have burned copies of the bull. Luther was excommunicated and Holy Roman Emperor Charles V was told to kill him on charges of heresy. Charles summoned him to Worms (verms) to be examined. Luther was declared a heretic. Luther had thirty days to return home before being declared an outlaw. On the way back to Wittenburg Luther was kidnapped by his friend a prince of Germany. the prince took him to a castle where luther lived in safety and translated the New Testament into German. Because Charles was busy with foreign affairs Luther was not bothered for the remainder of his life which he spent mostly in Wittenburg.
Pius V issued a bull, Regnans in Excelsis, dated April 27, 1570, that declared Elizabeth I a heretic and excommunicated her.
In January 1521, the Pope Leo X excommunicated Luther. He was then summoned to appear at the Diet of Worms, an assembly of the Holy Roman Empire. He refused to recant and Emperor Charles V declared him an outlaw and a heretic. Luther went into hiding at Wartburg Castle. In 1522, he returned to Wittenberg and in 1525 married Katharina von Bora, a former nun, with whom he had six children.
You would only be considered a heretic if the pope happened to tweet a matter of faith and you disagreed with it. For example, if the pope sent out a message that the Blessed Trinity consists of the Father, Son and Holy Spirit, that is an article of faith which all good Catholics must believe. To deny that would make one a heretic. However, if he tweeted that cats make better pets than dogs, you would be free to disagree.
Leipzig Debates against Eck on the nature of the word, sacraments, and the power/ primacy of the Pope
Pius V issued a bull, Regnans in Excelsis, dated April 27, 1570, that declared Elizabeth I a heretic and excommunicated her.
Martin was excommunicated because he was trying to go against most of the teachings of the Catholic church. (Which was caused by the 95 Theses, which Luther posted in Germany), The Church finally excommunicated him in 1521, and Luther was later banished in 1522. On 15 June 1520, the Pope warned Luther that he risked excommunication unless he recanted 41 heretical sentences drawn from his writings, including the 95 Theses, within 60 days. Luther set fire to the Popes warning at Wittenberg on 10 December 1520, Martin Luther was excommunicated by Pope Leo X on 3 January 1521 for heresy and apostasy, and for leading others astray.
The Edict of Worms was issued on the 25th of May, 1521 by the Holy Roman Empire, Charles V and the Pope. It banned all the writings of Martin Luther. Labeled him a heretic and enemy of the state.