Luther, in fact, wrote a book called The Bondage of the Will in which he asserted that the fundamental issue of the Reformation, the basic difference between genuine Protestantism and Roman Catholicism, is this issue, whether the will of the natural man is bound or free. Calvinism shows itself as pure Protestantism by its confession concerning the will in the Westminster Confession of Faith:
Sa doktrinang ito, ganap na nagkakaisa sina Luther at Calvin. Sumulat pa nga si Luther ng isang akalat na pinamagatang The Bondage of the Will ("Ang Pagkaalipin ng Kalooban"), kung saan iginiit niya na ang pangunahing isyu ng Repormasyon, ang pangunahing pagkakaiba ng tunay na Protestantismo at Romano Catolicismo, ay ang isyung ito, kung ang alipin ba ng taong makalaman ay alipin o malaya.