The central message that missionaries share with people is the message of the Restoration. This restoration is referring to Christ's church on the earth.
When Christ was on the Earth He set up a church. This was not the first time His church was on the earth (nor would it be the last). But, as we know, Christ was rejected and crucified. He left His twelve apostles to govern the church. These men held the priesthood of God and through them Jesus Christ continued to run His church. Over time, these twelve men were rejected and killed. With them went God's holy priesthood. Without the priesthood on the earth Christ's church was lost. People set up their own churches and professed them to be the true church. Many of these pepole were honest, truth-seeking people attempting to do God's work. Much good has come and will continue to come from these churches. But without the priesthood of God, they could not act in the name of our savior Jesus Christ.
After nearly 2,000 years, our Heavenly Father showed His love to us by restoring that original church. God called a prophet through whom He restored that priesthood authority held by Christ's original apostles. There is a prophet on the earth today. He is assisted by twelve modern-day apostles. God loves us. He has restored His church. I know this to be true.