Girolamo Savonarola was a Dominican friar who lived in Renaissance Florence and strongly preached against the secularization of Florentine culture in the arts and corruption among the clergy. The "Red Shirts" are associated with Giusseppe Garibaldi during the struggle to unify Italy in the 1850s, and Benito Mussolini lived almost 500 years after Savonarola.

Martin Luther, in contrast to the Roman Catholic Church at that time, championed all of these ideas. In opposition to the idea of indulgences saving one's soul, Luther claimed that faith alone could do so. He opposed the idea that only the Pope and clergy could interpret scripture in favor of a more individualized approach. To facilitate this, he promoted the translation of the Latin Scriptures into the vernacular languages so that the common people could more readily access them.

Martin Luther, a German priest and theologian, wrote the 95 Theses in protest against many of the teachings of the Roman Catholic Church.

It was Friedrich Nietzsche who wrote, in Thus Spoke Zarathustra (1891), that "God is dead." Nietzsche is considered massively influential in the development of atheist/agnostic philosophical schools of thought in the 20th century. Karl Marx is the intellectual father of a Marxist understanding of economics (sometimes incorrectly conflated with Communism). Baruch de Spinoza was a 17th century Enlightenment philosopher. Adam Smith was a Scottish Philosopher and considered an intellectual founder of market capitalism. Immanuel Kant was a German rationalist philosopher of the 18th century.

During the Counter-Reformation, the Catholic Church responded to Martin Luther's critiques and the rise of Protestantism by clarifying their beliefs. Unlike Luther, the Catholic Church continued to believe that priests should not marry and should remain celibate.

As a condition of the marriage of the Grand Duke of Lithuania to the Queen of Poland, the Grand Duke had to convert to Catholicism. Following the marriage, the people of Lithuania were forced to convert as well as to avoid conflict between the two nationalities over the issue in the future.

Poland had largely converted to Catholicism over the course of the the 13th century, but at the time of the Union of Kreva in 1386 Lithuania was still largely a pagan nation. With the two nations joined under one royal family, and the Lithuanian Grand Duke converting to Catholicism as part of the marriage, conflict over what religion the people of Lithuania would partake in caused much conflict.

Deism was a concept that admitted that God's existence was obvious to all who observed nature; however, it called into question the need for organized religion as Deism prompted the idea that in order to be close to God one should go out into nature and connect with God's creations. 

Angered by the Pope's refusal to grant him a divorce from his wife, Henry VIII declared that England would leave the Catholic Church and would now be part of the newly created Church of England, with Henry as the head of the church. He subsequently granted himself a divorce from his wife and took a new bride.

John Calvin was influential in reforming the church as a pastor in Geneva, Strasbourg, and Worms. Martin Luther was a church reformer who predated Calvin, and was primarily responsible for the development of Protestantism.