An indulgence is essentially a 'get out of purgatory free card' that the Catholic Church sold, allowing people to buy remission of punishment for sins without having to suffer penance, which Luther saw as fundamentally inconsistent with Christian teaching

Henry VIII wrote or published a book called 'The Defense of the Seven Sacraments' (actually written by Thomas More) in response to Luther's attack on the sacraments, demonstrating how controversial this issue was across European rulers.

Luther's second great work, 'The Babylonian Captivity of the Church' (October 1520), is written in Latin for educated clerical audiences rather than the vernacular, representing a different rhetorical strategy aimed at converting learned clergy to the priesthood of all believers doctrine.

Luther initially did not intend to provoke the political and military dislocation associated with the Protestant Reformation; he was an extremely conservative thinker whose intention was to cleanse the church of borrowings and traditions lacking scriptural justification

Luther's famous doctrine 'sola fide, sola scriptura' (by faith alone and by scripture alone) means that for a human to be justified in God's sight, saved, or blessed, the only way this can happen is through faith and scripture in an Augustinian sense, not through works, rituals, or accumulated church traditions

Luther lived from 1483 to 1546, and his most noted achievement was nailing the 95 Theses on the cathedral door in 1517

Luther was an extraordinarily devout individual whose sins were relatively mild (unlike Augustine's), and he became an Augustinian friar due to his powerful religious faith

Luther's 1521 statement to the Diet of Worms—'Here I stand, I can do no other, you must prove me wrong by scripture or I will not accept the proof'—demonstrates his powerful, independent conscience and his refusal to accept ecclesiastical or political authority without scriptural justification

Luther spent considerable time studying scripture before publishing the 95 Theses, particularly writing on Romans and the Psalms, emphasizing faith as the key to salvation, particularly Romans 1:17 ('he who through faith is righteous shall live')

Luther was contemptuous of reason and the Aristotelian tradition, believing that reason was the devil's and that Aristotle and scholasticism had led Christianity astray and done great evil to Christian believers

Luther's theological work is enormous, and he produced a large body of publications on a wide variety of biblical texts, making him prolific in the manner of German philosophers

The printing press was a relatively new invention crucial to the Protestant Reformation's success, as it allowed Lutheran doctrine to be disseminated widely while the Church could neither suppress all copies nor eliminate all printing presses, unlike earlier reform attempts

Luther attacks the papal claim to sole authority for interpreting scripture, arguing that all Christian believers should have access to scripture and that restricting access to an authoritative self-appointed group of interpreters is both unscriptural and unchristian

In 'The Babylonian Captivity of the Church,' Luther attacks the Catholic sacramental system, initially concluding that only three of the seven sacraments are valid (Lord's Supper, Baptism, Confession), and eventually reducing this to only two (Lord's Supper and Baptism)

Earlier reformers attempted to reform Catholicism from within before Luther, but they failed continuously in the previous century, making Luther the decisive breakthrough figure.

Protestantism makes its greatest inroads in northern Germany, the further from Rome, because the closer you are to Rome, the easier it is for papal authorities to keep people in line and enforce orthodoxy.

In 'The Freedom of a Christian Man' (1520), an open letter to Pope Leo written in Latin (later translated to German), Luther argues that without faith man is completely egocentric and incapable of free will, and that only God's inscrutable grace enables freedom.

Luther's idea of having only a good will as the basis for morality was later extrapolated and developed into Kant's moral theory, showing how Luther's theological ideas, while modified, contributed foundational concepts to German philosophy.

Luther moved away from an Aristotelian doctrine of virtue (that good habits lead to good character) and instead argued there is no necessary connection between faith and works—one can do good works and be evil, or be evil but receive God's grace at death and be saved.

In his preface to the German Bible translation, Luther articulates the hermeneutical principle 'do not make a Moses out of Christ'—meaning that Christ adds something new to the Mosaic law and transcends it rather than merely replicating it.