1 John 1:4 And these things write we unto you, that your joy may be full.
John had just said that they (the Apostles and eyewitnesses) were declaring to us the things that they had seen and heard concerning that eternal life (Jesus), for the purpose that we might fellowship with them and with the Father. Now here he says the Apostles are writing these things about Eternal Life for another purpose: that our joy may be full. The significant promise is this: when we read and hear the Scriptures, that testify of Jesus, we will be full of joy.
In a world of sin and a life of suffering we need a huge injection of joy. And the promise says we receive just this every time we hear the Gospel, receive the Sacrament, and remember our Baptism. This is a deep, abiding, inner joy, that nothing and no one can take away. This promised joy surfaces in the felt emotions of the Christian quite often, and it often shows in the eyes and smiling face of believers.
But not always; the joy is not always felt and the expressions do not always show it. We have worries and cares, anger and frustration, doubts and fears, sad times and bad times that cloud over and suppress the genuine joy that is always there. For this reason we need to believe the promise: we are joyful, we have the joy of the Lord. It has not left us; other emotions, deceits of the mind, and worldly struggles have crowded it out. Our cup is always running over; we are just in need of reminders. Therefore, we go to church, attend Bible classes, and spend time alone with God. And even, or especially, when we are not feeling good we pay attention to the Gospel of Joy; we respond with prayer, thanks, and praise. The Joy will return to our feelings, and even if it doesn’t right away we still believe it is there. Listen to “these things” that your joy may be full.