Put off concerning the former [way of life] the old man, which is corrupt.
A Christian has a war going on inside him all the time. The old nature is constantly battling the new. We are exhorted in this verse to continually practice keeping the old nature under control. And it is SO hard to do. Why? Because the old nature is corrupt. It can do nothing to please God. In fact, it takes us away from Him. We must keep our old nature down so that the new nature will guide and control us. John says in his letter that the new nature CANNOT sin. Which nature would you rather have dominate your life, one that is corrupt or one that cannot do wrong?