“Why do we need to receive our endowment in the temple before serving a mission?” New Era, Feb. 2014, 39
Both anciently and in modern times, the Savior sent forth His servants into the world after they were “endowed with power from on high” (see Luke 24:49; D&C 43:15–16; 105:33). We receive this blessing through the ordinances and covenants of the temple.
Elder D. Todd Christofferson of the Quorum of the Twelve Apostles has explained that “this ‘power of godliness’ [D&C 84:20] comes in the person and by the influence of the Holy Ghost.”1 Temple ordinances and covenants give us power, protection, and gifts through the Spirit. The Lord wants to bless you with these things whether you serve a mission or not. But if you have chosen to serve a mission, He especially wants to ensure that you have these blessings before you go because, among other things, your ability to teach and testify as a missionary will be enhanced through these blessings and through what you are taught in the temple.
In addition, as a missionary you will teach people about and invite people to receive the ordinances and covenants of the temple. It therefore makes sense for you to have received the ordinances you will be inviting people to work toward as they come unto Christ.