First, it is great because this joy is good news for all the people, for everyone; no one is left out. Before the angel said that this good news was for them, he said it was for everyone, even them; even you!
Then, the angel said that this good news was a Person, the Son of David, a Savior, Christ the Lord. In saying that, the angel said it all. The long-awaited and promised, Deliverer, Anointed, Eternal God, Himself entered His creation as a baby boy. This was good news of great joy because of Who had come and what He would do. This is the key to experiencing the great joy of the gospel; knowing your Savior, Jesus Christ the Lord.
There are some who equate joy with happiness, which comes and goes, dependent upon the weather (conditions). The great joy of the gospel is not dependent upon conditions. That is why the book of James can say, “Count it all joy when you encounter various trials, knowing….” You can experience the great joy of the gospel in a great trial, difficulty, or disaster if you know what you can know in the midst of it. Many Christians are not experiencing this great joy because they have not learned what they can know in every situation and so they do not know the first thing of the gospel, the great joy it has in it!
Some are only joyful if their prayers get answered, or people are nice to them, or they receive some blessing of some kind. But the great joy of the gospel is also when prayers go unanswered, when people are rude, crude, and down-right mean to you, and even when you are in a “blessing drought;” even then you can experience great joy, if you know the One who is with you and what He is doing in you, with you, through you, as you, in the world around you! Joy, Joy, Joy!
Jesus said in His prayer (He allowed John to listen in on it) in John 17:13, that He was praying so that His disciples could have His joy fulfilled in them. The great joy of the gospel is the answer to Jesus’ prayer. It is great because it is a great prayer request of Jesus and it if for everyone, who will receive it.
To summarize: The great joy of the gospel is a Person, knowing Jesus Christ the Lord as your Savior and the work that He is doing in you, regardless of the conditions. He is with you and is at work in you, with you, through you, as you, in the world around you; no matter what is happening around you. His joy is yours because of the work He is doing in you, with you….His work is bring out His life from within you for the glory of God the Father. He gets the glory and you get the Joy!
What a deal. What a God. What a Savior. What a season to celebrate.