In this verse it talks about something that I think often times we misunderstand. Sometimes we believe that just the act of doing missionary work cleanses us from sin. I don't think that it true. Saying it that way turns missionary work into a contract. Rather, and I believe this is true with all the commandments, as we DO the gospel, as we are obedient, conversion happens within our hearts. And as we are converted we experience what the people of King Benjamin experienced. "We have no more disposition to do evil, but to do good continually." When we are converted the conditions are made right in our hearts to seek out a remission of our sins, but we still have to repent, we still have to come unto Christ.