Charles Ellicott Commentary James 5:20

"let him know, that he who converteth a sinner from the error of his way shall save a soul from death, and shall cover a multitude of sins." — James 5:20 (ASV)

Let him know. Or, as it rather seems to be, Know; be absolutely sure of this, in a knowledge better than all the Gnostic and Agnostic learning of the day. He who turns a sinner from the error of his way shall save a soul from death—the means for this being given him by the Savior of all—and shall hide a multitude of sins; not, of course, his own sins, but those of the penitent, brought back by this good servant into the fold.

So it is possible to be a fellow-worker with Christ (2 Corinthians 6:1) and a sharer in His work of salvation, as, in another sense, we too vicariously suffer for the sins and faults of others (Compare to Colossians 1:24, and Butler’s Analogy, Part 2, Chapter 5).

What St. James was in word, he also was in deed, for he “prayed fervently” for the pardon and conversion of those who killed him. “Hold,” said some of the bystanders. When the martyr sank upon the stones: “The Just is praying for you!” Stephen’s prayer won Saul for the infant Church: it can hardly be that James’s last breathings of pity were unanswered by God.