The context of this verse is very important in order to understanding what John was pointing to. First, John was preaching a baptism of repentance (Matthew 3:2). Second, John refused to be promoted by others or confuse himself with Christ, but was just “the voice of one crying out in the wilderness, ‘Make straight the way of the Lord’” (John 1:23). Third, John makes the analogy between Jesus as a “bridegroom…who has the bride” (the church), and John as “the friend of the bridegroom” (3:29). How then can John say “Therefore this joy of mine is now complete” and how will Jesus increase through John decreasing?
Allow me to quote from the Chinese brother, Witness Lee: “By the context of these verses (3:16-21) we may know that all who come to the Lord to be redeemed by Him are the Lord’s bride, and the Lord is the Bridegroom. John the Baptist is not this Bridegroom; he is the friend of the bridegroom. It is the Lord who is the Bridegroom who will have the bride.
Following this, the next verse says, “He must increase.” What does this mean? This simply means that Christ will have a bride, and the bride is His increase.” To see Christ increase through His bride made John’s joy complete.
The core character of any leader in the church must be that Christ’s life increases in us. Paul put it in these words; “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me” (Galatians 2:20). This is how leadership was prepared for the Early Church and must be so today. There is no other way to have our joy complete!