This is a vast topic. Essentially, the Reformation began as an attempt to **reform** the Catholic church, and when the latter showed itself incapable of reforming itself (at least at the time) Martin Luther and others issued a solemn "Protest" in 1529 and relectantly broke away to form their own church. Later of course further divisions followed. (The word "Protest" gave rise to the nickname "Protestant" for the reformers). Please also note that there have been earlier attempts to reform the Catholic Church, for example by Wycliffe (in England) and Hus (in Bohemia and Moravia). The Catholic Church reacted with the utmost savagery at the time.