Luther suffered in the monastery from what he called Anfechtungen; he felt he was too sinful to be saved and experienced extreme anxiety over his relationship with God and his fate. His confessor at Erfurt, Johann von Staupitt, recommended that he read St. Augustine and pursue a PhD. He was sent to the University of Wittenberg permanently in 1511, where he earned his doctorate in theology in 1512. He started teaching Aristotelian philosophy and eventually was promoted to professor of Bible. In 1515 he began a lecture series on the New Testament Book of Romans that exhibited ideas which would later become staples in his theology, such as the imputation of righteousness, disputing theologians like Occam and Biel. His mature, more radical theology was likely only formulated several years later.