See: D&C 121:34
To be called is to be a member of the Church and kingdom of God on earth; it is to be numbered with the saints; it is to accept the gospel and receive the everlasting covenant; it is to have part and lot in the earthly Zion; it is to be born again, to be a son or a daughter of the Lord Jesus Christ; to have membership in the household of faith; it is to be on the path leading to eternal life and to have the hope of eternal glory; it is to have a conditional promise of eternal life; it is to be an inheritor of all the blessings of the gospel, provided there is continued obedience to the laws and ordinances thereof.
Within this over-all framework, there are individual calls to positions of trust and responsibility, but these are simply assignments to labor on the Lord’s errand, in particular places, for a time and a season. The call itself is to the gospel cause; it is not reserved for apostles and prophets or for the great and mighty in Israel; it is for all the members of the kingdom. (Bruce R. McConkie, Doctrinal New Testament Commentary, 3:326.) — Doctrine and Covenants Student Manual, p. 298
President N. Eldon Tanner interpreted this passage [D&C 121:34-40] as referring to those who fail to magnify their priesthood or who use it as it should not be used: “I know of many cases where a man has gradually failed to magnify his priesthood and moved away from activity in the Church. As a result, a man who has been very active loses his testimony and the Spirit of the Lord withdraws from him, and he begins to criticize those in authority, and to persecute the saints, apostatize, and fight against God.” (In Conference Report, Apr. 1970, p. 52.)
President Tanner later said: “We must not be nearly dependable, but always dependable. Let us be faithful in the little things, as well as the big ones. Can I be depended upon to fill every assignment, whether it be for a two-and-a-half minute talk, home teaching, a visit to the sick, or a call as a stake or full-time missionary?
“Remember, . . there are many called, but few are chosen. And why are they not chosen? ‘Because their hearts are set so much upon the things of this world, and aspire to the honors of men, . . .’ (D&C 121:34–35), and they are not dependable.” (“Dependability,” Ensign, Apr. 1974, p. 5.) — Doctrine and Covenants Student Manual, p. 299