11 [My] brethren, do not speak evil about or accuse one another. He that maligns a brother or judges his brother is maligning and criticizing the Law and judging the Law. But if you judge the Law, you are not a practicer of the Law but a censor and judge [of it]. 12 One only is the Lawgiver and Judge Who is able to save and to destroy [the One Who has the absolute power of life and death]. [But you] who are you that [you presume to] pass judgment on your neighbor? James 4:11-12 [AMP]
I am pretty sure that this passage is primarily talking about the kind of inner-church debates that too often degenerate into name-calling and even the questioning of one another’s Christian convictions. James’ justification for his prohibition is interesting -> to speak against or to judge one’s brother or sister is to speak against or to judge the law. The law to which James refers, in light of his usage of the word elsewhere in the letter, is the Old Testament law.
16 You shall not go up and down as a dispenser of gossip and scandal among your people, nor shall you [secure yourself by false testimony or by silence and] endanger the life of your neighbor. I am the Lord. Leviticus 19:16 [AMP]
How is it that ‘judging’ a fellow believer involves sitting in judgment on this law? Since James contrasts ‘judging the law’ with keeping the law, he apparently thinks that failure to ‘do the law’ involves an implicit denial of the law’s authority. However high and orthodox our view of God’s law might be, a failure actually to do it says to the world that we do not in fact put much respect to it.
For verse 12 I am going to copy the commentary I read on this topic. I don’t want to take credit for my learning on this one.
Speaking evil of fellow-Christians is wrong not only because it involves ‘judging the law’; it is wrong also because it involves ‘judging the neighbor’. And this critical, condemnatory judgment involves both disobedience of the demand that we love the neighbour and an arrogant presumption on the rights of God himself. For he is the one Lawgiver and Judge who alone has the right to determine the eternal fate of his creatures (cf. also Matt. 10:28). Yet when we criticize and condemn others, we are in fact pronouncing our own verdict over their spiritual destiny.
Tyndale New Testament Commentaries: James (2015)