The tongue is another test of living faith because the genuineness of a person’s faith will inevitably be demonstrated by his speech. The tongue only produces what it is told to produce by the heart, where sin originates.
Like fire, the tongue’s sinful words can spread destruction rapidly, or as its accompanying smoke, those words can permeate and ruin everything around it.
Three illustrations from nature—spring, fig tree, saltwater—demonstrate the sinfulness of cursing. The genuine believer will not contradict his profession of faith by the regular use of unwholesome words.
It might be said that a person’s speech is a reliable measure of his spiritual temperature, a monitor of the inner human condition.
Excerpt from James: Guidelines for a Happy Christian Life by John MacArthur