I was present at a discourse that the prophet Joseph delivered on baptism for the dead 15 August 1840.1 He read the greater part of the 15th chapter of Corinthians and remarked that the Gospel of Jesus Christ brought glad tidings of great joy, and then remarked that he saw a widow in that congregation that had a son who died without being baptized, and this widow in reading the sayings of Jesus "except a man be born of water and of the spirit he cannot enter the kingdom of heaven," and that not one jot nor tittle of the Savior's words should pass away, but all should be fulfilled.
He then said that this widow should have glad tidings in that thing. He also Said the apostle was talking to a people who understood baptism for the dead, for it was practiced among them. He went on to say that people could now act for their friends who had departed this life, and that the plan of salvation was calculated to save all who were willing to obey the requirements of the law of God. He went on and made a very beautiful discourse.
1. The occasion of this discourse was the funeral of Seymour Brunson, a prominent member of the Church in Nauvoo. The introduction of the doctrine of ordinance work for the dead changed the face of Mormonism and while it was slow in building, became the foundation of one of the major efforts of the Church. It is possible that Joseph Smith was preparing the ground for this in his sermon of 8Apr40. Joseph Smith announced that a temple should be built in the following conference of the Church (see 3Oct40(2)). After the temple announcement, the following day he preached again on the subject of baptism for the dead (see 4Oct40).
Two women who were present at the discourse left statements at the "History Office, Great Salt Lake City, Nov. 29th, one-half past 10 o'clock a.m."
Jane Neymon states that Joseph preached Seymour Brunson's funeral sermon and then first introduced the subject of baptism of the dead, and said to the people, "I have laid the subject of baptism for the dead before you. You may receive or reject it as you choose," Sept. 13th (Aug. 15th) she then went and was baptized for her son Cyrus Livingston Neymon by Harvey Olmstead. Joseph on hearing of it at table in the evening, asked what he said, on his telling what the ceremony was it proved that father Olmstead had it right
Vienna Jaques witnessed the same by riding into the river on horseback to get close so as to hear what the ceremony would be. [Statement in "Addresses" Joseph Smith Collection, Box 4 folder 4, LDS Archives.]
See also 27Mar42.