Nephi's endorsement of journal writing is found in the final words of the first verse, which, when read differently, essentially says,: "I make a record of my proceedings in my days because I have had a great knowledge of the goodness and the mysteries of God." If you have had a knowledge of the goodness and the mysteries of God, you should make a record of your proceedings in your days. Your posterity will benefit from hearing about your experiences and relationship with God.
