James spoke about the inconsistencies of religious people:
With the tongue we praise our Lord and Father, and with it we curse men, who have been made in God’s likeness. Out of the same mouth come praise [or blessing] and cursing. My brothers, this should not be. Can both fresh water and salt water flow from the same spring? My brothers, can a fig tree bear olives, or a grapevine bear figs? Neither can a salt spring produce fresh water. (James 3:9–12 niv)
James combined two pictures there. One is of a tree. He said that a fig tree will never bear another kind of fruit, such as olives. The kind of tree indicates the kind of fruit that it will bear. The tree, in that picture, is the heart; the fruit is the words that come out of the mouth. The second picture James used is that of a spring of water. He said that if brackish, salty water comes out of a spring, you know the water in the spring is salty.
The two trees represent two natures. A tree that produces fruit different from what it should produce is corrupt; that tree represents the old man. A good tree is the new man in Jesus Christ. The old man cannot bring forth good fruit. Jesus said that clearly many times. (See, for instance, John 15:1–8.) Out of that old, carnal nature will always come fruit that corresponds to that nature.
The fountain, or spring, represents something spiritual. A pure spring is the Holy Spirit. A corrupt, salty, impure spring is another spirit.