Why do James and Peter in their respective epistles both make statements that claim that some human action or another can "cover a multitude of sins" (James 5:20, 1 Peter 4:8)? If we are reading straight through the Bible, by the time we've come this far (almost to the end), we've pretty much been convinced that the only way that sins are "covered" is by the blood of Christ's sacrifice. We're acutely aware that we have no hope apart from Him and His imputed righteousness, and we've been warned many times from thinking that we can somehow purchase our own redemption. But now, here at the end of the book - a mere two flips of the page distant from each other - are a pair of statements that irritate my mind, which likes everything to be clean and simple. In trying to solve this mystery, I think that the 1 Peter passage is the easiest to explain. It is more or less a paraphrase of Proverbs 10:12.
1 Peter 4:8 - Above all, keep loving one another earnestly, since love covers a multitude of sins.
Proverbs 10:12 - Hatred stirs up strife, but love covers all offenses.
It is easier to see the intended meaning of the word "covers" when we see it juxtaposed with what hatred does. If you hate someone, conflict is going to keep flaring up whenever the two of you are together, because the tiniest offenses are going to be amplified. If you truly love someone, however, you overlook most of that person's faults, and you try to help them through the larger ones. Think of all of the things that love does not do according to 1 Corinthians 13:4-6 and you'll quickly understand how love can "cover all offenses" or "cover a multitude of sins". It's not that the love of the lover is somehow meritorious of atonement for the lover's sins, but his love excuses (or "covers") the sin of the one that he loves. In other words, we are talking about a "covering" that is horizontal - taking place between human beings - not vertical - taking place between man and God.
The James passage is a little more difficult to decypher. We are told in James 5:20 that if a person tries to correct his brother or sister in Christ and ends up bringing that wandering individual back to the truth, that such a person has "saved his soul from death" (we understand that it's Christ who has actually saved the person, but his brother has brought him back to the faith) and "will cover a multitude of sins."
The whole key to understanding the right meaning of the passage has to hinge on whose sins are being "covered" by this action: those of the one who is brought back, or those of the one who brought him back. If we take the latter approach, then we are going to end up with a kind of sacrament that can truly cover sin: bringing back lost sheep to the fold. But if we understand the sins being covered as belonging to the one who is brought back, then everything smooths out. Of course a multitude of his sins are covered - he has returned to faith in Christ! Christ's blood covers his sins, not the act of restoring him to the truth. That act has simply showed him the way back.
So we don't seem to actually have any meritorious works here in these two examples after all, but the fact that the Holy Spirit chose to use the same words that we normally associate with salvation does indicate the absolute importance of the two ideas. Loving one another and bringing wandering souls back to the truth are two of the most Christ-like actions that we can enjoy. And they're both intimately related as well. If we're not doing one, we're not doing the other.