There is a difference between saying and doing. For example, if I were to tell you that I am a tree, you would think I am crazy. I don't have bark. I don't have leaves. I can also type this. Trees have bark and leaves, and cannot type a coherent message on a laptop. Our ability to draw a distinction between a human and a tree is also applicable to our spiritual walk with God.
There are many people who call themselves Christians and follow Christ wholeheartedly. There are also many people who call themselves Christians, come to church on Sunday, and live like the devil for the rest of the week. John the apostle makes a clear distinction between the two.
John makes it clear that a person can claim to be in the light, yet still be in darkness. How is this possible? Simple, someone can claim to be a Christian, yet not follow Christ. They can do this in the same way that I can call myself a tree, yet not be a tree.
I am only a tree if I have every essential quality of a tree (bark, leaves, etc.) I do not have these qualities. Therefore, I am not a tree. I have every essential quality of a human being. Therefore, I am a human being. In the same way, I can only be a Christian if I walk daily in a personal relationship with Jesus Christ. This relationship will change your life. If our lives have not changed, we need to question where we are with God. Only then can we begin to make progress.