Back to the passage in question...
James 4:11-12 KJV
(11) Speak not evil one of another, brethren. He that speaketh evil of his brother, and judgeth his brother, speaketh evil of the law, and judgeth the law: but if thou judge the law, thou art not a doer of the law, but a judge.
(12) There is one lawgiver, who is able to save and to destroy: who art thou that judgest another?
I came across this from Expositors Bible Commentary...
In part and a little wordy...
"Speak not against one another, brethren." The context shows what kind of adverse speaking is meant. It is not so much abusive or calumnious language that is condemned, as the love of finding fault. The censorious temper is utterly unchristian. It means that we have been paying an amount of attention to the conduct of others which would have been better bestowed upon our own.
It means also that we have been paying this attention, not in order to help, but in order to criticise, and criticise unfavourably. It shows, moreover, that we have a very inadequate estimate of our own frailty and shortcomings. If we knew how worthy of blame we ourselves are, we should be much less ready to deal out blame to others.
But over and above all this, censoriousness is an invasion of the Divine prerogatives. It is not merely a transgression of the royal law of love, but a setting oneself above the law, as if it were a mistake, or did not apply to oneself. It is a climbing up on to that judgment-seat on which God alone has the right to sit, and a publishing of judgments upon others which He alone has the right to pronounce. This is the aspect of it on which St. James lays most stress.
"He that speaketh against a brother, or judgeth a brother, speaketh against the law and judgeth the law." St. James is probably not referring to Christ’s command in the Sermon on the Mount. "Judge not, that ye be not judged. For with what judgment ye judge, ye shall be judged". (Mat_7:1-2) It is a law of far wider scope that is in his mind, the same as that of which he has already spoken, "the perfect law, the law of liberty"; "the (Jas_1:25) royal law, according to the Scripture, Thou shalt love thy neighbor as thyself". (Jas_2:8) No one who knows this law, and has at all grasped its meaning and scope, can suppose that observance of it is compatible with habitual criticism of the conduct of others, and frequent utterance of un-favorable judgments respecting them. No man, however willing he may be to have his conduct laid open to criticism, is fond of being constantly subjected to it. Still less can any one be fond of being made the object of slighting and condemnatory remarks.
Every man’s personal experience has taught him that; and if he loves his neighbor as himself, he will take care to inflict on him as little pain of this kind as possible. If, with full knowledge of the royal law of charity, and with full experience of the vexation which adverse criticism causes, he still persists in framing and expressing unfriendly opinions respecting other people, then he is setting himself up as superior, not only to those whom he presumes to judge, but to the law itself. He is, by his conduct, condemning the law of love as a bad law, or at least as so defective that a superior person like himself may without scruple disregard it. In judging and condemning his brother he is judging and condemning the law; and he who condemns a law assumes that he is in possession of some higher principle by which he tests it and finds it wanting.
What is the higher principle by which the censorious person justifies his contempt for the law of love? He has nothing to show us but his own arrogance and self-confidence. He knows what the duty of other persons is, and how signally they fall short of it. To talk of "hoping all things, and enduring all things," and of "taking not account of evil," may be all very well theoretically of an ideal state of society; but in the very far from ideal world in which we have to live it is necessary to keep one’s eye open to the conduct of other people, and to keep them up to the mark by letting them and their acquaintances know what we think of them. It is no use mincing matters or being mealy-mouthed; wherever abuses are found, or even suspected, they must be denounced. And if other persons neglect their duty in this particular, the censorious man is not going to share such responsibility. This is the kind of reasoning by which flagrant violations of the law of love are frequently justified.
And such reasoning, as St. James plainly shows, amounts really to this, that those who employ it know better than the Divine Lawgiver the principles by which human society ought to be governed. He has clearly promulgated a law; and they ascend His judgment-seat, and intimate that very serious exceptions and modifications are necessary; indeed, that in some cases the law must be entirely superseded. They, at any rate, are not bound by it.
This proneness to judge and condemn others is further proof of that want of humility about which so much was said in the previous section. Pride, the most subtle of sins, has very many forms, and one of them is the love of finding fault; that is, the love of assuming an attitude of superiority, not only towards other persons, but towards the law of charity (Love) and Him who is the Author of it.
To a truly humble man this is impossible. He is accustomed to contrast the outcome of his own life with the requirements of God’s law, and to know how awful is the gulf which separates the one from the other. He knows too much against himself to take delight in censuring the faults of others. Censoriousness is a sure sign that he who is addicted to it is ignorant of the immensity of his own shortcomings. No man who habitually considers his own transgressions will be eager to be severe upon the transgressions of others, or to usurp functions which require full authority and perfect knowledge for their equitable and adequate performance.
Censoriousness brings yet another evil in its train. Indulgence in the habit of prying into the acts and motives of others leaves us little time and less liking for searching carefully into our own acts and motives. The two things act and react upon one another by a natural law. The more seriously and frequently we examine ourselves, the less prone we shall be to criticize others; and the more pertinaciously we busy ourselves about the supposed shortcomings and delinquencies of our neighbors, the less we are likely to investigate and realize our own grievous sins. All the more will this be the case if we are in the habit of giving utterance to the uncharitable judgments which we love to frame. He who constantly expresses his detestation of evil by denouncing the evil doings of his brethren is not the man most likely to express his detestation of it by the holiness of his own life; and the man whose whole life is a protest against sin is not the man most given to protesting against sinners.
"One only is Lawgiver and Judge, even He who is able to save and to destroy." There is one and only one Source of all law and authority, and that Source is God Himself. Jesus Christ affirmed the same doctrine when He consented to plead, as a prisoner charged with many crimes, before the judgment-seat of His own creature, Pontius Pilate. "Thou wouldest have no power against Me, except it were given thee from above". (Joh_19:11) It was Christ’s last word to the Roman Procurator, a declaration of the supremacy of God in the government of the world, and a protest against the claim insinuated in "I have power to release Thee, and I have power to crucify Thee," to be possessed of an authority that was irresponsible. Jesus declared that Pilate’s power over Himself was the result of a Divine commission; for the possession and exercise of all authority are the gift of God, and can have no other origin. And this sole Fount of authority, this one only Lawgiver and Judge, has no need of assessors. While He delegates some portions of His power to human representatives, He requires no man. He allows no man, to share his judgment-seat, or to cancel or modify His laws.
It is one of those cases in which the possession of power is proof of the possession of right. "He who is able to save and to destroy," who has the power to execute sentences respecting the weal and woe of immortal souls, has the right to pronounce such sentences. Man has no right to frame and utter such judgments, because he has no power to put them into execution; and the practice of uttering them is a perpetual usurpation of Divine prerogatives. It is an approach to that sin which brought about the fall of the angels.
"But who art thou, that judgest thy neighbor?" St. James concludes this brief section against the sin of censoriousness by a telling argumentum ad hominem. Granted that there are grave evils in some of the brethren among whom and with whom you live; granted that it is quite necessary that these evils should be noticed and condemned; are you precisely the persons that are best qualified to do it?
Putting aside the question of authority, what are your personal qualifications for the office of a censor and a judge? Is there that blamelessness of life, that gravity of behavior, that purity of motive, that severe control of tongue, that freedom from contamination from the world, that overflowing charity which marks the man of pure religion?
To such a man finding fault with his brethren is real pain; and therefore to be fond of finding fault is strong evidence that these necessary qualities are not possessed. Least of all is such a one fond of disclosing to others the sins which he has discovered in an erring brother. Indeed, there is scarcely a better way of detecting our own "secret faults" than that of noticing what blemishes we are most prone to suspect and denounce in the lives of our neighbors.
It is often our own personal acquaintance with iniquity that makes us suppose that others must be like ourselves. It is our own meanness, dishonesty, pride, or impurity that we see reflected on what is perhaps only the surface of a life whose secret springs and motives lie in a sphere quite beyond our groveling comprehension. Here, again, St. James is quite in harmony with St. Paul, who asks the same question: "Who art thou that judgest the servant of another? to his own lord he standeth or falleth…But thou, why dost thou judge thy brother? or thou again, why dost thou set at naught thy brother? For we shall all stand before the judgment-seat of God?". (Rom_14:4; Rom_14:10)