“Finally, my brothers, rejoice in the Lord. To write the same things to you is no trouble to me and is safe for you.” (3:1 ESV)
Joy is a strong theme in Philippians. Christians are to be joyful people. That is something that has been neglected or misunderstood by believers throughout history. We either have the old stereotype of the dour, sour Christian who could find the negative in anything; or the more contemporary mentally vacuous, emotional Christians ever chasing the next “high.” The former saw faith as a series of rules that everyone tended to get wrong. The later seems to see faith as a feeling devoid of understanding.
True joy is not an emotion exactly. It is a state of mind, a choice. In our current condition it has a bit of a bitter sweet aspect to it. There is a lot of longing (sehnsucht) in joy. It is a desire for the pleasure of something that we know, that we sometimes taste, but that we treasure all the more because we know what it is like to not have it. In the case of Jesus’ followers, we rejoice in Him. We have had the experience of life without Christ, so we know the treasure that life is with Him. We also know that life in this fallen world is such that we do not get to experience His presence fully all the time the way that we one day will. The tastes we get are enough to give us a tremendous desire. That is the feeling of joy that we cultivate.
That is why it is not a feeling even though it evokes feelings. Joy is something that we practice. Look at the cases where Paul expresses joy or calls for it here in Philippians: he prays for other believers with joy (1:4) he rejoices in the gospel’s proclamation, even under false pretenses (1:8) growth in faith is related to joy (1:25) Christian unity brings it (2:2) he will rejoice in the face of suffering (2:17) as the believers ought also to rejoice with him in that prospect (2:18) etc. Joy is a discipline independent of good times or bad. It is not happiness or even contentment.
It is the exercise of choosing that which will bring us pleasure, and it is something we are to practice at all times.