 He became convinced that the church had lost sight of what he saw as several of the central truths of Christianity — the most important being the doctrine of justification by faith alone.He began to teach that salvation is a gift of God's grace through Christ received by faith alone.As a result of his lectures on the Psalms and Paul's letter to the Romans, from 1513–1516, Luther "achieved an exegetical breakthrough, an insight into the all-encompassing grace of God and all-sufficient merit of Christ."