Christian Joy Is... 
Supernatural Origin
Beyond natural power or comprehension
It originates from the Lord
"God is the ultimate source of joy and we attain joy through [connection with God]"
A call to stand in God's presence
Anchored in Jesus Christ
The spiritual sharing in unfathomable joy
It is divine and yet experienced by humans
The same joy found in the heart of Jesus Christ
The Meaning of Christianity
Simply the "ability to be joyful"
"Christianity is, by its very nature, joy--ability to be joyful," — Joseph Ratzinger
"A Christian, is a man and a woman of joy . . . The Christian sings with joy, and walks, and carries this joy." — Pope Francis
The Epicentre of the Scriptures
The nativity story begins with an announcement of joy
"So Gabriel went into the house and said to her, 'Joy be to you, favoured one! the Lord is with you.'" Luke 1:28 (WNT)
The teachings of Christ are intended to grant us joy
"These things I have spoken to you, that my joy may be in you, and that your joy may be full." John 15:11 (ESV)
A Gift From God
It can also be seen a grace which we must seek
"Rejoice in the Lord always; again I will say, rejoice." Philippians 4:4 (ESV)
Like the heart of Jesus, the Holy Spirit, and the grace of God, it fills us from within.
Creates the "overwhelming" effect we all have when we experience it.
Tied to Truth
Irrepressible*
"Only when love and truth are in harmony can man know joy." — Joseph Ratzinger
"God disposes the mind and heart of His creature to meet joy, at the same time as truth." — Pope Paul VI
May refer to how we momentarily forget troubles and hardships when joyful
Eternal
Christians should rejoice in their sufferings for Christ and never let it be suppressed by evil.
"Not only that, but we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God's love has been poured into our hearts through the Holy Spirit who has been given to us." Romans 5:3-5 (NIV)
Humans do not create joy
It is a gift from God, and because it is also divine, it is everlasting
Christians will share in eternal joy with the heavenly beings after Christ returns, worshipping Him.
"And all the people shouted with a great shout when they praised the LORD, because the foundation of the house of the LORD was laid… many shouted aloud for joy." Ezra 3:11-12 (ESV)