Martin Luther arrived at his earthshaking conclusions imbued with biblical exposition. As a professor, he taught the book of Psalms verse by verse from 1513 to 1515, Romans from 1515 to 1516, Galatians from 1516 until 1517, the book of Hebrews from 1517 to 1518 and then the Psalms again from 1519 until 1521.