Pope Leo X excommunicates Martin Luther in the papal bull Decet Romanum Pontificem.

Pope Leo X was head of the Catholic Church and ruler of the Papal States from 9 March 1513 to his death in December 1521.

Martin Luther was a German priest, theologian, author, hymnwriter, professor, and former Augustinian friar. Luther was the seminal figure of the Protestant Reformation, and his theological beliefs form the basis of Lutheranism. He is widely regarded as one of the most influential figures in Western and Christian history.

A papal bull is a type of public decree, letters patent, or charter issued by the pope of the Catholic Church. It is named after the leaden seal (bulla) traditionally appended to authenticate it.

Decet Romanum Pontificem is a papal bull issued on 3 January 1521 by Pope Leo X to effect the excommunication of German theologian Martin Luther and some of his colleagues—notably Andreas Karlstadt—for refusing to recant forty-one of Luther's Ninety-five Theses, threatened by the earlier papal bull Exsurge Domine. Luther had burned his copy of Exsurge Domine on 10 December 1520 at the Elster Gate in Wittenberg to indicate his response. The title Decet Romanum Pontificem comes from the first three Latin words of its text.

January 3 is the third day of the year in the Gregorian calendar; 362 days remain until the end of the year.