This is a simple description of the process that has brought the people to this point. They have experience with Benjamin as their king, and are able to have faith in him, based on that experience. That initial willingness to believe carries them through the message to the revelation of the Spirit. Both in the words of Benjamin and the outpouring of the Spirit they have received "great knowledge." As with other occurrences of the word joy in the Book of Mormon, they are describing a state of spiritual joy.