Pope Leo X identified forty heretical statements in the writings of Luther. In the bull Exsurge Domine, Leo X gave Luther sixty days to recant of these heretical statements.
On the sixtieth and last day of the probationary period, Luther publicly burned a copy of the bull. Technically, Luther excommunicated himself by not complying.
In response, Leo X issued Decet Romanum Pontificem on January 3, 1521, making the excommunication of Luther official.