This entire chapter is making one point. We will end there, but first observe how James maps over realities from the physical world to human nature. What comes out of our mouths affects the whole person, just like a bridle does in the mouth of a horse (3); just as a rudder does a ship (4); just as a small fire sets aflame a great forest (5).
Here is a great irony: mankind has tamed every animal, but not our own tongues (8-9). It is a “restless evil, full of poison.” And yet it was never meant to be this way (10). Again an analogy from nature: springs only give forth one kind of water (11). The same with fruit: fig trees don’t produce olives (12).
If we are reborn, regenerated, made new in Christ, James says, then be who you are. He’s not saying to olive trees, “Become fig trees.” That would be impossible and cruel. He’s saying to fig trees, “Be fig trees.”
But we cannot do it (8). Thus we need wisdom from above (15), that will solve our real problem, which begins upstream from the mouth, in the heart. Make the spring pure, and so will be the stream. And what pollutes our spring so often is twofold: jealousy and selfish ambition (14 & 16). These map over quite closely to the first and tenth commandments (see Exodus 20).
This wisdom from above - the gospel - produces God’s intended harvest: righteousness, and therefore peace between people.