Mosiah 28:3

"Now they were desirous that salvation should be declared to every creature, for they could not bear that any human soul should perish; yea, even the very thoughts that any soul should endure endless torment did cause them to quake and tremble."

The sons of Mosiah, once the vilest of sinners, knew that they were saved from torment through the gospel of Jesus Christ.  As their souls were saved, they grew in desire for all people to accept the gospel and be saved.  They became great missionaries because they cared for the people they taught and truly wanted to save them!