The book of James deals largely with how we, as Christians, should live. Part of righteous living involves controlling our tongue. What we say is important. If we call ourselves Christians, we not only have a responsibility to imitate Christ in our actions. We also have a responsibility to imitate Him in our manner or speech.
James stresses this in his epistle. He points out that if we think of ourselves as obedient followers of Christ, but do not know how to control our speech, there are two negative outcomes. First, our beliefs about Christ do us no good. Second, we deceive ourselves.
First, simply knowing about Christ do us little good. There is a fine line between knowing about Christ and knowing Christ. Those who know Christ will try to imitate Him. We can believe certain things about Christ (know about Him) all day long. However, it does us no good if we do not know Christ and imitate Him. The act of speaking is an action. Because of this, we must also imitate Christ in our speech, or else we are not truly being obedient to Christ in all of our actions.
Second, if we do not control our tongues, we deceive ourselves. We may truly believe that we never sin. That does not mean that it is true. In fact, to believe so opens a door to the sin of pride. In the same way, we may believe that all we say is Christ-like, but that does not make it so. We can become self-delusional. We may convince ourselves that there is nothing wrong when, in fact, there may be.
How should we then speak? When it comes to our Christian brothers and sisters, we should encourage them to continue in the faith during their struggles. We should build them up and help them grow.
When it comes to non-believers, we should not be harsh in our speech toward them. We should always speak the truth to them in love.