Spirits can only be revealed in flaming fire or glory. Angels have advanced further, their light and glory being tabernacled; and hence they appear in bodily shape. The spirits of just men are made ministering servants to those who are sealed unto life eternal, and it is through them that the sealing power comes down.
Patriarch Adams is now one of the spirits of the just men made perfect; and if revealed now, must be revealed in fire; and the glory could not be endured. Jesus showed Himself to His disciples, and they thought it was His spirit, and they were afraid to approach His spirit. Angels have advanced higher in knowledge and power than spirits.
["History of the Church, 6:51, Joseph Smith, Sunday, October 8th 1843; TPJS, p. 325; Concepts Quotations Dealing With the Origin, Identity Position of Adam Eve"]