Jamieson, Fausset and Brown on James 5:20

20. Let him--the converted.

know--for his comfort, and the encouragement of others to do likewise.

shall save--future. The salvation of the one so converted shall be manifested hereafter.

shall hide a multitude of sins--not his own, but the sins of the converted. The Greek verb in the middle voice requires this. Pr 10:12 refers to charity "covering" the sins of others before men; James to one's effecting by the conversion of another that that other's sins be covered before God, namely, with Christ's atonement. He effects this by making the convert partaker in the Christian covenant for the remission of all sins. Though this hiding of sins was included in the previous "shall save," James expresses it to mark in detail the greatness of the blessing conferred on the penitent through the converter's instrumentality, and to incite others to the same good deed.