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For the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit.
Rom. 14:17 NKJV
Joy is that deep, supernatural fulfillment that comes in knowing that we are experiencing and expressing the one who is true satisfaction, Jesus Christ. Joy begins with acknowledging that we are unconditionally loved, graciously forgiven, and eternally kept in Christ. Joy is released in our lives when we cultivate Christ’s conscious, constant presence.
Joy is not produced by emotional highs: supernatural fulfillment is imparted by obedience to God’s commands. Joy is not dependent on pleasant circumstances, but it is the fruit of finding and meeting Christ in the midst of all our trials both pleasant and painful. Joy is renewed by worshiping the risen Jesus and by sharing him with others. Jesus Christ is the one person who can bring true joy.
Joy is something very deep and profound, something that affects the whole and entire personality. In other words it comes to this; there is only one thing that can give true joy and that is a contemplation of the Lord Jesus Christ. He satisfies my mind; He satisfies my emotions; He satisfies my every desire. He and His great salvation include the whole personality and nothing less, and in Him I am complete. Joy, in other words, is the response and the reaction of the soul to a knowledge of the Lord Jesus Christ.
Martyn Lloyd-Jones, Fellowship with God (Crossway Books, 1992).