Why do we do missionary work for the Gospel of Jesus Christ here upon the earth?
It was the last thing that Jesus Christ himself commanded of His followers as he ascended to his Father as recorded in Matthew 28:
“Go ye therefore and teach all nations, baptizing them in the name of the Father, and the Son, and of the Holy Ghost.”
That should be enough for all of us who believe that the Church of Jesus Christ of Latter-Day Saints is His gospel here upon the earth to guide us. It offers truth and answers. But people make their own decisions. Our job is just to teach, testify, share, and serve.
We don’t always do this, but we try.
But in addition we have some very clear direction from modern day apostles and prophets here upon the earth that help us make right decisions as we do our best to live a better life.