That Augustine was an influence on Martin Luther is undeniable; different historians and theologians, however, vary in opinion as how great this influence actually was. Luther joined the Order of Saint Augustine at Erfurt in July 1505, and received a spiritual formation that focused on “Great Father Augustine” (which also was the title of a hymn then in Augustinian use).
The Order used Augustine’s thought in the theological preparation given to its candidates. One of Augustine’s superiors, Johann von Stauptiz O.S.A. (1468-1524), gave Augustine especial emphasis. It is certain, therefore, that Luther had read and studied many of Augustine’s writings, that he memorised passages from Augustine, and that he cited Augustine more than any other non-Scriptural source.
It is known, for example, that Luther used a copy of the printed collection of some works by Augustine that had been published in Strassburg by Martinus Flach in 1489 under the title of Opuscula plurima, for Luther with his own hand wrote annotations on its margins in 1509. And in 1516 Luther was known to have been studying the eighth volume of the Opera Omnia (the world's first complete printed collection of Augustine's works) edited by Johannes Amerbach in eleven volumes in Basel in 1506
Specialist studies of Luther’s writings have determined the wide range of Augustine’s works that Luther cited. Luther was one of the first major figures to have readily available to him the ‘entire Augustine’ in the Johann Amerbach printed edition of Augustine’s Opera omnia ("Complete Works") mentioned above. Luther was not only trained in a theology that was heavily Augustinian, but also found resonance in Augustine’s thought for some of the theological issues with which he himself struggled, e.g., sin, grace, predestination, the interpretation of Scripture, and faith.
He initially made his own the basic tenets of Augustine’s theology. This is most evident in his work as a professor at the University of Wittenberg before the time he posted his now-famous ninety-five theses on the castle church there on 31st October 1517. Luther encouraged his fellow professors to read Augustine’s works. Augustine was the patron saint of the university. With the public eruption of the Protestant Reformation in 1517, Luther did not abandon Augustine, but used him selectively and sometimes out of context in an effort to support the changed direction of his own line of thought.
There became large areas of thought where Luther diverged from Augustine, e.g., in matters regarding the authority and magisterium of the Church. Even so, Augustine’s thought was still frequently used as the base from which Luther’s theology proceeded. As Luther matured, his theology became increasingly independent of Augustine, but he continued to praise Augustine. Further historical and theological research remains to be done on Augustine’s influence on Luther at various stages of the latter’s life.
In the development of his theology, Martin Luther turned to the writings and thought of Augustine more than to any other individual source except the Bible. In Augustine of Hippo (354-430) there was a firm position on many of the issues that were to become the focus of controversy during the Protestant Reformation. Indeed, for this reason Augustine has been regarded as a determinative church source for the theology that constituted the Protestant Reformation.
It has been demonstrated that Luther began studying Augustine as early as 1509, when he was twenty-six years of age and in his third year as a member of the Order of Saint Augustine. He quickly became enthralled by the writings of Augustine. Luther became thoroughly acquainted with them. This was not merely because he was a member of the Order of Saint Augustine, but rather because of the intrinsic value and truth he found in them. Later he wrote, "I do not defend Augustine because I am an Augustinian. Before I began reading his works he meant nothing to me." The theology of Augustine was held in great esteem by Luther, and promoted widely while he was at Wittenberg. Luther even described Augustine as a leading advocate for reform in the church. Even so, further specialist study needs to be done on the thought of Luther at various periods of his life, for there were times when he used Augustine's writings possibly only because they were a convenient resource. There are also instances wherein Luther quoted Augustine quite selectively to suit his own purpose, and suggested that Augustine resonated with Lutheran reasoning when the fuller context of Augustine's writing in fact would demonstrate that this was untrue.
As Luther grew older and more independent in his thought, Luther turned less to Augustine. Even in his early writings at the time when the Reformation erupted, Luther did not merely reproduce Augustine's thought. It is true to say that sometimes the germ of Luther's ideas were already present in Augustine's writings, but it is not correct to attribute to Augustine more credit for Luther's own thinking than that. Even in the writing of the most mature Luther, Augustine was often the starting point, but only that.
The remainder at this page briefly examines some of the key theological differences between Augustine and Luther.
The visible and invisible church
The theology of Augustine about the visible and invisible church had great repercussions on the development of the thought of Luther. The invisible and true church comprised the whole company of the predestined. And because faith is invisible and unable to be measured, the logical conclusion for Luther was that the genuine church was invisible. Luther readily accepted this theology. It enabled him to come to terms with the discrepancies between the ideal church as presented in Scripture and the tainted visible church he saw around him. Although the church is made holy in heaven, the true church is hidden from the world.
The world understands the church on this earth as made up of sinners and saints, of hypocrites and pious believers, and of weeds and wheat. But the true church, which is the community of believers and the elect of God, is hidden in the visible church. This is the concept of communio sanctorum (the communion of saints), which Luther and also John Calvin largely adopted from his study of Augustine.
His development of the doctrine of Grace (gratia in Latin) and salvation was largely indebted to Augustine. Luther accepted the concept of operative grace, and agreed that God initiates the first grace toward passive man, just as a woman is passive when she conceives. This operative grace is the actual divine help that on occasion prompts the human will, in its internal act, to change from willing evil to willing good. In this the will is human passive.
On the other hand, in reference to those external acts wherein the human will is active, grace is called co-operative, and is also classified as being either actual or habitual. Luther agreed wholeheartedly with Augustine on the principle of sola gratia ("grace alone"), that only the grace of god was necessary for salvation. This grace was a totally free gift that God could choose to give, and it was neither possible nor necessary that a person could do anything relevant so as to seek it or to pay for it in some way.
The position of Luther on free will has its source in Augustine. Luther’s concept of free will was that, "the human will is like a beast of burden. If God rides it, it wills and goes as God wills; if Satan rides it, it wills and goes as Satan wills. Nor can it choose its rider, nor betake itself to him it would prefer, but it is the riders who contend for the possession." Luther presumed that this analogy has its source in Augustine, although his authorship of it is now disputed.
The doctrinal position of Luther on predestination and original sin was greatly influenced by what Augustine wrote. He completely agreed with Augustine on the principle of sola gratia, that a person reached heaven solely by the grace of God, and without any necessary or possible contribution on the part of the person concerned. Luther wrote that human pursuits after salvation and righteousness are unable to cause election, and therefore man receives no merit. The human will has no ability to thwart the sovereign purposes of God.
For the elect, proclaims Luther, predestination is "the sweetest of all doctrines," but to carnal persons it is "the bitterest and hardest of all." For Luther predestination and original sin were essential theological tenets, ensuring comfort and hope for the believer who had been rescued by the Sovereign grace (gratia in Latin) of God.
In summary, it has been said that the mainline Protestant Reformation can be considered an acute "Augustinianization" of Christian belief, a hallowing of the theological thought of Augustine more extensively than the Roman Catholic Church has done either before the Protestant Reformation or afterwards.Link
Was Luther Augustinian? This blog says that this question is difficult to answer from a historical perspective because more concrete evidence must be documented to show that Luther was influenced by Augustine. http://evangelicalcatholicmissionalfaithful.blogspot.com.au/2007/02/was-luther-augustinian.html