Some different reasons exist as to the need for baptism in the case of our Lord, he being without sin and in need of no repentance. His expression to John was, ‘Suffer it to be so now: for thus it becometh us to fulfil all righteousness.’ (Matt. 3:15.)
Nephi explains that Christ did fulfill all righteousness in being baptized in that: 1. He humbled himself before the Father; 2. He covenanted to be obedient and keep the Father’s commandments; 3. He had to be baptized to gain admission to the celestial kingdom; and 4. He set an example for all men to follow. (2 Ne. 31:4–11.)
Our Lord’s baptism ‘showeth unto the children of men the straightness of the path, and the narrowness of the gate, by which they should enter, he having set the example before them.’ (2 Ne. 31:9.) If even the King of the kingdom could not return to his high state of preexistent exaltation without complying with his own eternal law for admission to that kingdom, how can any man expect a celestial inheritance without an authorized and approved baptism?
(Bruce R. McConkie, Mormon Doctrine, p. 71)