Joseph defines the "more sure word of prophecy" as a promise from God that one will have eternal life.
Joseph Smith, "Discourse, 21 May 1843, as Reported by James Burgess," [8], The Joseph Smith Papers website,. accessed September 8, 2021
First chapter second Epistle of Peter. The first four verses are the preface to the whole subject. There are three grand Keys to unlock the whole subject. First what is the knowledge of God, second what is it to make our calling and election sure Third and last is how to make our calling and election sure. Ans[wer], it is to obtain a promise from God for myself that I shall have Eternal life, that is the more sure word of prophycy.