I have heard several people in church misquoting James 5:20 as meaning that a person’s sins will be “covered” if they convert another person. I think this is a gross misinterpretation of that scripture.
The confusion comes in whose sins are referenced and how those sins will be covered. The scripture says, “He that converteth the sinner from the error of his way...” This clearly means that because of the conversion you have taken the sinner off a path filled with inevitable future sins and put him on a different path that does not include those sins. By so doing you, “Shall save a soul from death, and shall hide a multitude of sins.” It is the sins of the converted person being covered not those of the missionary.
1 Peter 4:8 says that, “Charity shall cover a multitude of sins.” Are we to assume then that everyone who practices charity is also exempt from repentance? If this was true, we would have little need for a savior; we could simply save ourselves through missionary work and charitable acts. It seems much more logical that the reference in both scriptures is about changing the paths we and others are on and thereby eradicating or “cover[ing]” future sins.
There is nothing more sacred in the gospel than the process of repentance. It involves God’s own perfect Son suffering unimaginable pain and ultimately dying on the cross so that we may repent. To say that there is some other way to remove past sin than through the atoning blood of Jesus Christ is blasphemous.
Helaman 5:9
…yea, remember that there is no other way nor means whereby man can be saved, only through the atoning blood of Jesus Christ, who shall come…
(see also Mosiah 3:17 and Alma 38:9)