The Catholic Church developed absolute power during the Middle Ages and was ruthless in defending the faith.  As in most human institutions, absolute power bring corruption.  Hard to defend doctrines and teachings such as  indulgences, purgatory, Original Sin, Heaven and Hell, created untenable positions and necessitated strange and totalitarian practices.
Torture was used frequently during the Inquisition to obtain confessions of heresy. In the 1200s Pope Innocent IV officially sanctioned the practice.   The church has since apologized.    
The requirement of faith, enforced by centuries of enslavement by the church led to a bazaar history and one that would indicate that the Church can not be the true religion of Christ, but simply another institution in the history of man.

Man’s inhumanity to Man can occur in any time.  Religious zeal must be balanced. A nice and helpful question to ask from our own generation is WWJD - What Would Jesus Do?

Martin Luther was a German Augustinian Monk in the 1500's and was largely responsible for starting what is known as the Protestant Reformation.  He had a history of depression and was obsessed with guilt, confession and salvation.  He could find no peace through acts of good works, petition of the saints, confession or penance, the normal methods of the Church for salvation.  He could not do enough to purge his sins.  At the heart of it was his view of God of consuming fire damning souls to Hell and of Christ the Avenger.   
In 1510 Luther made a pilgrimage to Rome and became totally disillusioned by the ignorance, irreverence, frivolity, incompetence and immorality of the Italian clergy. 

Some of the following is paraphased from the book "Martin Luther, Here I Stand"  by Rowland H. Bainton.  Indulgences were designed for “release from the temporal punishments one must suffer in this life or in purgatory for the sins a person has committed."  The sale of indulgences developed into big business for the Church and led to major corruption.  The money was used to finance the Crusades, construction of Cathedrals, and “indulgences” of the clergy.  A Dominican friar named Tetzel, an associate of Luther, is credited with composing the popular rhyme for selling indulgences:
            “Remember that you are able to release them, for
              As soon as the coin in the coffer rings,
                  The soul from purgatory springs.” 
All of this outraged Martin Luther.  However, he still could not shake his demons, and the obsessive-compulsive need to confess and inability to “feel saved“.     
Johann von Staupitz was a Vicar of Luther’s Order and his spiritual guide and mentor.  Staupitz was also a mystic and tried to help Luther.  He instructed Luther about preparation and overcoming the ego, and releasing to become one with God.  Luther was partially successful, but the sense of alienation would return - the “dark night of the soul” of the mystics.
Staupitz then told Luther to study for the Doctor’s degree  and be a teacher.  Luther went back to studying the scriptures and discovered his new theology.  He found Christ who suffered with humanity and “came to love the Suffering Redeemer who could rout out the devils and demons”.
 For Luther, it was the mystery of the cross, unmerited Grace,  and faith alone, which are gifts of God; and that one can do nothing to obtain salvation.  Christ reconciled the wrath and mercy of God.  And this appeared to be the theology of St. Paul.     

Luther famously nailed his 95 theses on the door of the Castle Church in Wittenburg, Germany in 1517.  The statement attacked the practices of selling indulgences and the Pope's ability to grant the release of those souls in purgatory.  All of this helped launch the Protestant Reformation.  Luther was sincere, devout and pious and this gave him the courage to go up against the most powerful organization in his world, and would not “recant” the charges of heresy.  This is reflected in the famous injunction that is attributed to Luther: “Here I stand”.  Luther was eventually excommunicated.  

John Calvin, a French priest also broke for the Church and moved to Switzerland around 1530 to escape the religious violence between the Catholics and Protestants. In Geneva, Calvin joined the Reformation. During the 1540’s, his supporters gained control of positions in the city and it developed into a theocracy, where the priests rule in the name of god.

Calvin’s ideas about predestination and the depravity of man, came from Augustine of Hippo, St Augustine, a Catholic Church Father ~ 400 AD. Augustine developed concept of Orginal Sin: “from the evil use of free will (Eve) there arose depraved origin from a corrupt root”. God’s pre-knowledge knows who will be save and who will be damned. Augustine also developed ideas about the Trinity.

Calvin wrote: “miserable ruin into which the rebellion of the first man cast us into depravity and corruption” and even the newborns “indeed, their whole nature is a seed of sin”. Everthing came back to the fall of man and brought a depressing world-view. This view also allowed for more violence as in the 30 year war, burning heretics. Geneva allowed no music or dancing. No tolerance. Calvin had his share of his own depravity.

Of interest to Unitarians, Michael Servetus, Spanish priest and polymath, was condemed and burned at the stake. in Geneva, 1553.

He had written about errors of the Trinity, rejected Original Sin and Predestination.