After establishing the great power of the tongue (James 3), James further explores how using one’s tongue inappropriately has consequences. Speaking evil of a brother is closely tied with judging that brother, which is a job that God has not given His children. The Greek word used here for “speak evil” refers to slander, which is “the sin of those who meet in corners and gather in little groups and pass on confidential tidbits of information which destroy the good name of those who are not there to defend themselves.” (William Barclay, The Letters of James and Peter, The New Daily Study Bible, 128.) This is not a good use of the abilities God has entrusted.
There’s a higher consequence here, though. Not only is the evil speaker judging their brother, but they are speaking evil of and judging the law. Jesus already made it clear that His followers are not to judge one another (Matt. 7:1–5). Furthermore, speaking evil is also not loving one’s neighbor as oneself, which is a key part of summing up the entire law (Matt. 22:38–40). Sometimes someone can think the law doesn’t apply to them because they’re breaking it for a good reason. In this way, they place themselves above the law, judging it as unfair, unreasonable, or not worthy to be wholly followed. This changes one’s relationship with the law from a doer to a judge, which is not humanity’s place.
God is the only one who can give the law and judge people, because He’s the only one with the right to do it (as both Creator and Redeemer), and He’s the only one who does it correctly. “Who are you to judge another?” James asks, appealing to His followers’ lack of credentials for such an action.
James wraps up this section with an expanded definition of sin: “To him who knows to do good and does not do it, to him it is sin” (James 4:17). Lack of doing can be just as sinful as doing. It was a sin for the priest and the Levite to not help the man on the road (Luke 10:25–37). Notice that there is knowledge involved: “ . . . who knows to do good.” This was a theme in Jesus’ parables, too, as in the unfaithful servant who was punished more severely because he knew his master’s will (Luke 12:47). With increased understanding comes increased responsibility and greater consequences; simultaneously, with increased understanding comes increased potential for blessing others and having a closer intimacy with God. As His children allow the Holy Spirit to teach them, God will also guide them in what He has already revealed, leading them into doing more good as they learn more good.