How great a forest is set ablaze by such a small fire! And the tongue is a fire, a world of unrighteousness. The tongue is set among our members, staining the whole body, setting on fire the entire course of life, and set on fire by hell.  (James 3:5b-6)

The word ‘tongue’ translates γλῶσσα glossa which, as in English, means both the organ of speech and speech itself, in other words, language.

But of course, James is using a figure of speech, synecdoche (derived from the Greek synekdekhesthai from syn ‘with’, ek ‘out’ and dekhesthai ‘to receive’) where a part of represents the whole.  So, in his metaphor, the tongue represents the person.

Jesus uses synecdoche, but with the heart as the figure, in this rebuke of the Pharisees:

‘You brood of vipers! How can you speak good, when you are evil? For out of the abundance of the heart the mouth speaks. The good person out of his good treasure brings forth good, and the evil person out of his evil treasure brings forth evil. I tell you, on the day of judgement people will give account for every careless word they speak,for by your words you will be justified, and by your words you will be condemned.’  (Matthew 12:34-37)

And for Solomon, it is the mouth:

The mouth of the righteous is a fountain of life,
    but the mouth of the wicked conceals violence. (Proverbs 10:11)

All the above quotes are concerning speech being an indicated of the health or disease of a person’s soul.  But James continues to also point out the power of speech.  Having warned against the increased judgment for setting oneself as an authority on Scripture, James says this:

For we all stumble in many ways. And if anyone does not stumble in what he says, he is a perfect man, able also to bridle his whole body. If we put bits into the mouths of horses so that they obey us, we guide their whole bodies as well. Look at the ships also: though they are so large and are driven by strong winds, they are guided by a very small rudder wherever the will of the pilot directs.So also, the tongue is a small member, yet it boasts of great things.  (James 3:2-5a)

The three examples of the horse, a ship and wildfire, illustrate the sphere of influence, personal, our immediate circle and beyond.  A single rider might tame his own horse, while a pilot can steer his companions to safety or to their doom by shipwreck; and history can provide examples of inflammatory oratory that can take a country to war and ultimate ruin.

And thus, James would have us know that from mouth can frame blasphemy or benediction:

With (the mouth) we bless our Lord and Father, and with it we curse people who are made in the likeness of God. From the same mouth come blessing and cursing.  (James 3:9-10)

Likewise with the Apostle Paul, who would enjoin us to speak deliberately and considerately…

 Let no corrupting talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear.  (Ephesians 4:29)

… seeing that the message of grace is the Gospel:

For ‘everyone who calls on the name of the Lord will be saved.’ How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching?And how are they to preach unless they are sent? As it is written, ‘how beautiful are the feet of those who preach the good news!’  (Romans 10:13-15)

Proclaiming the Gospel is a commandment from Jesus to his followers:

‘Go into all the world and proclaim the gospel to the whole creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.’ (Mark 16:15-16)

Eternal life is God’s gift, and how any respond is between the hearer of the Word and God; nevertheless, the disciple is charged and held accountable if the Gospel is false, Paul again:

If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. (Galatians 1:9)

Or, if the Gospel is withheld.  Here, Jesus uses the analogy of salt:

You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people’s feet.  (Matthew 5:13)

So, we see that:

Death and life are in the power of the tongue,
    and those who love it will eat its fruits.  (Proverbs 18:21)

Scripture repeatedly draws attention to regulating the tongue by choosing our words carefully and opting not to speak anything that does not inform, encourage or bless. The last word then from James, who in proverbial format says, 

let every person be quick to hear, slow to speak (James 1:19)