The introductions demonstrate how Luther understood the Psalter as a Christ-centered book and how he used the psalms as the model for Christian prayer. Luther classifies each psalm as a psalm of prophecy, instruction, comfort, prayer, or thanksgiving and applies the message of the text to the life lived out under the cross. Luther also connects the psalms to the Ten Commandments and the petitions of the Lord's Prayer. For Luther, all aspects of the Christian's life, including the Psalms, relate to Christ and His Gospel of forgiveness, life, and salvation.