If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
Now, it is important that we make the distinction between joy and happiness, for joy is a quality of the spirit, whereas happiness is a quality of the emotion. So happiness is a variable, because it is related to the outward circumstances. Things are going great. I just got a new car. I'm so happy. I'm just whistling as you drive down the road. But I'm so preoccupied that I run into a tree, "Yikes." My happiness is gone. I'm miserable. I'm sad. I didn't have a chance to insure the thing yet. So, happiness is a variable; it can change very suddenly very dramatically.You may come and say, "Oh, I'm just having a horrible time and I don't know what I'm gonna do. I'm just loaded with debts and they are going to repossess all that I have. I don't know what I'm going to do." And so I sit down and write you out a check for ten thousand dollars and you say, "Oh, this is great." And it might make you so happy. Until you went and tried to cash the check, then you'd be sad again. So happiness is a variable related to the outward circumstances.But joy is a thing of the heart, the spirit, and it isn't a variable. It doesn't change; it's a constant. Because it is a joy that is related to my relationship with God, which is a constant. That relationship doesn't change, things may go bad, they may be horrible, but my relationship with God is secure, therefore I have the fullness of joy.John writes this epistle to bring you into that kind of a relationship with God, that you might have this fellowship with God. That your joy may be full.