Before Nauvoo, Joseph Smith received many revelations from the Lord, forming much of the Doctrine and Covenants we have to day. In the early days of the church, the Prophet Joseph Smith was receiving revelation about how the Lord’s church was to be run and organized. They addressed specific questions and restored doctrine. Like everyone though, Joseph did not learn everything all at once. It took time to study it out and to fully put all the pieces of a puzzle together. It seems that upon reaching Nauvoo, he started to see the whole picture. As a result, he began to expound on the further truths of the gospel. He didn’t just get up and tell the people “Thus saith the Lord,” but he taught true truths and doctrines.
This was only one reason that some of his earlier discourses were not recorded and saved as scripture. Very few of Joseph Smith’s sermons were recorded, and most of them were only small excerpts. In Nauvoo, we have the records of people writing down his words as he spoke, plus some letters from Joseph to congregations. These sermons contained true doctrine, which shed more light upon who we are, and our relationship to deity. As a result, they were and are considered scripture.
What I find interesting today is that we work in much the same way as the Nauvoo era. We don’t hear the prophet get up in conference very often and teach us brand new doctrine, or receive groundbreaking revelations. Instead he teaches us the truths of the God. As he does so by the Spirit, it is as though his words are scripture, for they are words of God.