Thompson writes to Kimball about Joseph preaching baptism for the dead (1840).
Robert B. Thompson letter, Nauvoo, Illinois, to Heber C. Kimball, London [England], November 5, 1840, Church History Library, MS 12476
You will have heard ere this of the death of our beloved Bishop Partridg Bro Seymour Brunson and the patriarch of the Church Joseph Smith Senr.
You will likewise probably have heard of the old doctrine of Baptism for the Dead which has been introduced by President Joseph Smith Jr. So that the Saints have the priviledge of being baptized for their relatives and friends who have not had the priviledge of hearing the gospel while in the flesh but who probably receive it while in the spirit in prison. so that they can claim them at the ressurrection of the just. this is certainly a glorious doctrine and shews forth the gracious purposes of our God, and the grandeour of that scheme which is to raise mankind from the ruins of the fall. Hundreds have allready Gone forth and been baptized for their friends who are deceased.