Joy in Paul's Teaching
To preface this essay, it is important to familiarize oneself with the story of Saul, the zealous persecutor of Christians and his transformation into the Apostle Paul (see Acts 9), as this is a large part of his reason for feeling such joy: "one of the main consequences of his reconciliation with God was his ability to 'rejoice in God through our Lord Jesus Christ' (Rom. 5:11) in spite of the afflictions, hardships and trials that came his way" (DPHL 511). According to W.G. Morrice, 131 of the 326 uses of the word for joy in the New Testament are ascribed to Paul. He also calls Paul the "theologian of joy as he undoubtedly was... of grace" and notes that both charis and chara in Greek are derived from the same root: char-
Paul's joy as a Christ-follower can be seen throughout his epistles, and this will focus on Philippians, joy as a fruit of the Spirit in Galatians, and joy in the midst of suffering or persecution.
Philippians
The letter of Philippians was probably written by Paul while he was in prison (or under house arrest) in Rome before AD 70, yet this letter is known for the theological theme of joy that runs throughout. According to G.F. Hawthorne, the joy described here is "a settled state of mind characterized by peace, an attitude that views life... with equanimity. It is a confident way of looking at life that is rooted in faith in the living Lord of the church" (DPHL 713).
He talks often of the joy he finds in the Philippians, writing that he always offers "prayer with joy in my every prayer for you all" (Phil. 1:4) because of their "participation in the gospel" which could mean their obedience to support his ministry as well as their maturity in Christ. Later, he asks them to "make my joy complete" by being unified {Phil. 2:2), which is another major theme in Philippians. Still later he reminds them to "Rejoice in the Lord always; again I will say, rejoice! Let your gentle spirit be known to all men. The Lord is near. Be anxious for nothing..." (Phil. 4:4-6). In a sense, he is giving them reasons to rejoice: First he wants them to show their joy (with gentleness) so that others may witness the hope they have. Next the fact that Jesus is near is pretty exciting- He is present and working in us! Therefore, they shouldn't have to worry, but rather rejoice and learn to be "content in whatever circumstances" (Phil. 4:11) because the man upstairs has got it all under control. But where on earth is this abnormal joy supposed to come from in a world that is pretty messed up? Well it's a gift...
The Fruit of Joy
So in Galatians 5:22 Paul describes what the fruits of the Spirit are, in contrast to the "deeds of the flesh" (Gal. 5:19). The fruits of the Spirit are as follows: love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control. This is supposed to be what overflows from the life of a Christ-follower as they slowly are transformed by the working of the Holy Spirit in their lives, which is Christ's Spirit. More of this sanctification process is discussed in Romans 8, and another good resource is A Celebration of Discipline by Richard Foster.
Joy in the midst of suffering...
"...I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us" (Romans 8:18). Paul was persecuted as a follower of the Way. He was beaten (Acts 16:19-24, Acts 18:17), imprisoned, mocked by intellectuals (Acts 17:18), under a death warrant (Acts 23:12), and eventually killed. In his letter to the Philippians, Paul wrote that during his imprisonment, others have been emboldened to preach, though some out of "selfish ambition rather than from pure motives" (Phil. 2:17), but that regardless he rejoices because Christ is proclaimed and exalted. So where does he find the motivation for all of this joy? Well not only does Paul wait in hopeful expectation for his chance to see God's glory and to be with Jesus and fellowship in His suffering, death and resurrection (Phil. 3:10-11). See, when he calls the Philippian believers "my joy and crown" (Phil. 4:1), he's reminding them that he is running his race to win a heavenly prize (y'know, storing up those treasures in heaven, away from all the mothballs).
works cited
"Joy" by W.G. Morrice in Dictionary of Paul and His Letters.
"Letter to the Philippians" by G.F. Hawthorne in Dictionary of Paul and His Letters.
New American Standard Bible