And they came to John and said to him, "Rabbi, he who was with you across the Jordan, to whom you bore witness--look, he is baptizing, and all are going to him." John answered, "A person cannot receive even one thing unless it is given him from heaven. You yourselves bear me witness, that I said, 'I am not the Christ, but I have been sent before him.' The one who has the bride is the bridegroom. The friend of the bridegroom, who stands and hears him, rejoices greatly at the bridegroom's voice. Therefore this joy of mine is now complete. He must increase, but I must decrease." - John 3:26-30
All that John had received had been given to Him from heaven. And his understanding of this, combined with his all consuming joy in seeing the purpose of his life and ministry alive and glorious in the person of Jesus Christ is what enabled him to see everything in the light of eternity rather than through the polluted lense of the world. Jesus was everything to John, his entire purpose for being existed in the Son of God - Jesus Christ. The joy that John speaks of when he states, “Therefore this joy of mine is now complete” was a blessed reality to John even before he was born. The Holy Spirit filled John with an un-surpassing joy that we read of in a profound and joyous portion of scripture from the book of Luke.
When John claimed to his disciples that, “He must increase, but I must decrease” he in no way felt diminished or pushed aside. Instead, John felt the completion of his joy, knowing what we all must know; that we must decrease as we are brought from death to life in Jesus Christ - as the spirit of the Lord works in us to put to death the power of our sinful flesh and live in His righteousness by grace. We must always be in an increasing state of being conformed to the image of Jesus Christ.
Christ must increase and He will bring our joy to completion in Him.
For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. - Romans 8:29