Helaman 10

The Lord teaches us what it takes to have one's calling and election made sure or in other words, qualify for eternal life - 

"Blessed art thou, Nephi, for those things which thou hast done; for I have beheld how thou hast with unwearyingness declared the word, which I have given unto thee, unto this people. And thou hast not feared them, and hast not sought thine own life, but hast sought my will, and to keep my commandments."  Helaman 10:4)

Obviously "doing" is a condition of eternal life.