The imperative let him know means “he must know.” This “he” refers to the believer who turns the one who has gone astray back to the right way. The imperative is some sort of assurance and therefore may be rendered “remember this” (Good News Translation, Bible en français courant, New International Version) or “be sure of this” (New English Bible).

James describes the wandering away as the error of his way. Here the word error is from the same root as the verb “wander” in verse 19. The way, a parallel concept to “truth” in verse 19, is to be understood in an ethical sense, relating to a person’s life style (Psa 1.6; 101.2; Pro 4.14; see also the discussion on 1.8). In order to maintain the connection with verse 19, we may render the phrase the error of his way as “his wandering way.” This first sentence may also be expressed as “Be sure of this, whoever causes a sinner to stop wandering from the truth,” or “Be sure of this, whoever causes a sinner to begin listening to the true word again.”

Whoever turns a sinner back from his or her wandering will save his soul from death. The subject of will save is not God but a human person. In some languages it will therefore be more natural to use another word such as “rescue” (so Phillips, Revised English Bible) rather than “save.” The soul is not part of a person but represents the total person (see the discussion in 1.21). Death here apparently means more than physical death; it means more a spiritual and eternal death, since it is the result of sin (see the discussion in 1.15). There is a question as to whose soul is meant by his soul. Is it the person who has sinned? Or is it the person who has turned the sinner back? The text can be understood either way. But it seems clear that the soul that has been saved from death is the soul of the one who has sinned and then come back. In order to make this clear, we may have to repeat the word “sinner”; for example, “… will save that sinner’s soul from death” (Good News Translation, New Revised Standard Version). However, if we are not too sure about this interpretation and still believe that an alternative rendering should be provided, we can also render the phrase as “… will save his own soul,” as in the Good News Translation alternative rendering. Since the soul represents the total person, it may not need to be repeated in translation; for example, “… will save him from death” (New International Version), “will save them from death” (Contemporary English Version), or even “will rescue them from death.”

Turning back a sinner from wandering away will not only save that sinner, it will also cover a multitude of sins. This is the second assurance. The same phrase is used in 1 Peter 4.8. Here the verb cover is to be understood in the familiar Old Testament sense of “to cause to be forgotten” and therefore “to procure forgiveness” (Psa 32.1; Rom 4.7). The expression a multitude of sins is not meant to convey the degree of wickedness of the sinner but the extent of God’s grace and forgiveness (Psa 85.2). So the phrase may be rendered as “bring about the forgiveness of many sins” (Good News Translation) or “will cause many sins to be forgiven” (Translator’s New Testament). Since God is the one who forgives, it may be desirable to make this clear; for example, “will cause God to forgive the many sins that you have [or, that person has] committed.”

Here we also have the question as to whose sins are covered. Is it the sinner who is brought back? Is it the one who brings back the sinner? Or is it other people?
(1) One possible interpretation is to take the blessing of forgiveness as offered, not to any single person, but to both the person who brings back the sinner and the restored sinner, or to the Christian community at large. But this line of interpretation does not seem to fit the context and is not widely accepted.
(2) Another possibility is to take the assurance of forgiveness as offered to the person who brings about the return of a fellow believer. There are certainly some biblical texts that indicate that someone who brings others to repentance brings benefits and blessings to himself or herself (compare Ezek 3.18-21). It is a case of a good act rewarded. So understood, the phrase will cover a multitude of sins may be rendered as “will bring about the forgiveness of many of his own sins” (compare Barclay‘s rendering, “will draw a veil over a host of sins of his own”). It seems, however, a little strange that the assurance of forgiveness is not offered to the person who is saved.
(3) Both the context and the sequence of thought in this verse appear to favor a third possibility, namely that the assurance of pardon is offered to the person who is brought back. It is more natural to assume that both the assurance of salvation and the assurance of forgiveness are offered to the same person, namely the converted sinner. Reflecting this interpretation, and using inclusive language by adjusting the pronoun references, we may render verse 20 like this:
• You should know this: whoever among you turns sinners back from their wrong way will save them from death and bring about forgiveness of their many sins.
• Remember this: whoever among you causes a sinner [or, sinners] to stop wandering from the truth of the Gospel will rescue him [or, them] from death and will cause God to forgive his [or, their] many sins.

James is concerned about the well-being of the Christian community. And so, with a call for that community to take responsibility for their erring members, he concludes the letter.