Therefore each of you must put off falsehood and speak truthfully to his neighbor, for we are all members of one body.Remember, this passage is talking about True Change. When does a liar stop being a liar? When does an alcoholic stop being an alcoholic. When does the adulterer stop being an adulterer? These are the questions that need to be answered. Modern psychology will tell an alcoholic that he will always be an alcoholic. They tell him it is a disease that he has, but with the true changing of the Spirit of God, you can be made free from the laws of sin and death.
Here is the first example of the Put Off, Be Renewed, Put On, method of change. In this example we are dealing with a liar. You are to put off falsehood and you are to put on speaking the truth. Notice that the middle step is left out. I believe that the reason for this is because it is an assumption at this point. That middle step can only be accomplished if you are a true child of God. If you are, then the Spirit of God is at work within you, constantly renewing your mind.
But the passage does give a helpful little thought to establish that renewed mind. It tells us a reason why we shouldn't lie. We are members of one body. Now, we could get into all sorts of discussion about exactly what this means and is referring to. Think though, it would be damaging to the body if the brain told the knee to keep moving when the knee was feeling pain. If the brain said, "you'll be OK, keep going." There could be trouble. Or what if the knee said that it was OK when it really wasn't, the damage for the knee could be made worse or the knee could end up damaging the whole body if it were to give out in a bad circumstance. The point is that one part of the body lying to another part of the body just shouldn't happen.
When does the liar stop being a liar? Is it when he stops telling lies? No. It is when he becomes a speaker of the truth.