“So [important] is the tongue among our members, that it defileth the whole body, and setteth on fire the course of nature; and [or when] it is set on fire of Gehenna.”

Here, in strong, symbolic language, the apostle points out the great and bad influence  of  an  evil  tongue  —  a   tongue  set   on   fire   (figuratively)  by Gehenna (figuratively). For a tongue to be set on fire of Gehenna signifies that it is set going in evil by a perverse disposition, self-willed, selfish, hateful, malicious, the sort of disposition which, in spite of knowledge and opportunity, unless controlled and reformed, will be counted worthy to be destroyed — the class for whom the “Second death,” the real “lake of fire,” the real Gehenna, is intended. One in that attitude may by his tongue kindle a great fire, a destructive disturbance, which, wherever it has contact, will work evil in the entire course of nature. A few malicious words often arouse all the evil passions of the speaker, engender the same in others and react upon the first. And continuance in such an evil course finally corrupts the entire man, and brings him under sentence as utterly unworthy of life.