"The dead are not perfect without us, neither are we without them. We  have a mission to perform for and in their behalf; we have a certain  work to do in order to liberate those who, because of the ignorance and  the unfavorable circumstances in which they were placed while here, are  unprepared for eternal life; we have to open the door for them, by  performing ordinances which they cannot perform for themselves and which  are essential to their release fro the 'prison-house' to come forth and  live according to God in the spirit and be judged according to men in  the flesh."  
--Joseph F. Smith, April 11, 1878 J. D., 19:264
"To my mind, there isn't anything to great and so glorious in  this world as to labor for the salvation of the living and for the  redemption of the dead." 
-- Joseph F. Smith YWJ 23:130, January 1912 
"The work for our dead . . . l should not be neglected. We  should avail ourselves of those sacred and potent ordinances of the  gospel which have been revealed as essential to the happiness, salvation  and redemption of those who have lived in this world when they could  not learn the gospel and have died without the knowledge of it and are  not waiting for us; their children, who are living in an age when these,  ordinances can be performed, to do this work necessary for their  release from the prison-house. Through our efforts in their behalf their  chains of bondage will fall from them, and the darkness surrounding  them will clear away, that light may shine upon them and they shall hear  in thespirit world of the work that has been done for them by their  children here and will rejoice with you in your performance of these  duties. 
-- Joseph F. Smith CR, p. 6, October 1916