Recent readers of this blog may not be aware of my profound debt to Mormon theology and metaphysics; becuase I did most of my writing on this subject up to about five years ago, culminating in the 'mini-book' Speculations of a Theoretical Mormon.
If asked to pick the single most important achievement of Mormonism - I would pick the emphasis on marriage and the family as being at the heart of Life Everlasting in Heaven, and the basis of our theosis in mortal life.
This Christian insight was largely missed by mainstream Christianity for its first 1800 years (related to the systematic neglect and denigration of the Fourth Gospel); therefore requiring further major divine revelations primarily via the prophet Joseph Smith and some of his descendants in the Church of Jesus Christ of Latter Day Saints.
This major theological achievement - of putting eternal marriage and the family structure at the core of resurrected life, and of building this from basic metaphysical assumptions - is largely distinguishable from Smith's other and much better known attainments of producing The Book of Mormon, and of organising a church which has grown exponentially (at the same rate as mainstream Christianity) to about fifteen million current members.
The Mormon position on the family has been usefully summarised in a 1995 Proclamation.
This is a contribution to Christianity that I regard as core to my faith; and I have never seen it adequately expressed anywhere else than in Mormonism - except in the works of William Arkle, who independently, and by intuition, converged-on a very similar understanding (but more than a century later).
Since I greately value Arkle's work, this strikes me as an important confirmation. But the major confirmation of Mormon theology has been in my own life, heart, and thinking - ever since I first encountered and understood it back in 2008.