There are several things going on in James 3:6. Two of the more interesting are the placement of γλῶσσα and whether καθίσταται is a middle or passive.
“And the tongue is a fire (ἡ γλῶσσα πῦρ)! The tongue is a world of iniquity (ὁ κόσμος τῆς ἀδικίας ἡ γλῶσσα) set among our members (καθίσταται ἐν τοῖς μέλεσιν ἡμῶν ); it defiles the whole body (ἡ σπιλοῦσα ὅλον τὸ σῶμα), sets on fire the course of our life (καὶ φλογίζουσα τὸν τροχὸν τῆς γενέσεως), and is set on fire by hell (καὶ φλογιζομένη ὑπὸ τῆς γεέννης).”
If the second γλῶσσα goes with the preceding, it means “The tongue is a (or “the”) world of iniquity, and this means that the subject of καθίσταται is drawn from its personal ending.
If γλῶσσα goes with the following, then κόσμος τῆς ἀδικίας is a noun phrase in apposition to γλῶσσα (“The tongue is a fire, a world of iniquity”) and the second γλῶσσα is the subject of καθίσταται. “And the tongue is a fire. The tongue, a world of unrighteousness, is placed among our members” (CSB).
I will phrase this verse in a video to lay out it’s structure.
But the more significant challenge is the voice of καθίσταται.
If it is a passive, then who is the agent of the action? “The tongue is set among our members” (ESV). Satan? That gives Satan too much power in our human creation. Is it a divine passive? God does set the tongue as part of our body, but that seems tautological (“The tongue, a world of unrighteousness, is placed among our members,” CSB) . The NASB (see also NRSV) gets around this issue by saying, “the tongue is set among our members as that which defiles”; this places the emphasis of “is set” not on its placement in the body but on the goal of defiling. However, I am not sure you can get “as” out of the Greek. Both readings seem unlikely to me.
Rather, καθίσταται is a true middle. The NET is uncharacteristically dynamic — “And the tongue is a fire! The tongue represents the world of wrongdoing among the parts of our bodies” — but it does show one way to handle the middle, better than a reflexive translation like “sets itself.”
Two translations simply drop the verb altogether and let the flow of the sentence carry its meaning. “And among all the parts of the body, the tongue is a flame of fire. It is a whole world of wickedness” (NLT). “The tongue also is a fire, a world of evil among the parts of the body” (NIV). I think the word should be translated, which does require making an interpretive decision.
However specifically you handle the Greek, the meaning is clear. It is out of the heart that the mouth speaks, and hence the tongue is a tool for defiling the person. I read Matt 15:18–19 yesterday. “But the things that go out of the mouth come from the heart, and these defile the person. For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander.” Remember what “defile” means. It means the person is not acceptable to God, and as Christians we believe that it is the heart that defiles, and moves the tongue to speak and the hands to do evil.
But do we really believe that? Do we really believe that the angry words, the slanderous words, that so often come out of the human mouth truly make that person unacceptable to God, make him or her a defilement?
Slanderous words defile the speaker before God. We can probably all think of what that does not mean, but what does it mean? Words have meaning, and “defile” is a strong word.