The purest, highest, truest joy is to fill the Christian life like sunshine. Paul wrote this as a prisoner, as one who had been conformed to the death of Christ. Nothing is ever to dim our spiritual joy. Paul is not dwelling on the source of this joy, on all the streams of grace and blessings that produce this rejoicing.
He is rather picturing the life that is animated by the joy with which it shines. In 3:1 he calls on us “to rejoice in the Lord” even while we beware of errorists, knowing that we have the true righteousness and the hope of the blessed resurrection (3:9–11, 20, 21). Here he adds “alway,” no matter what the circumstances of our life may be. Our sun of grace is “always” shining.*
*R. C. H. Lenski, (on Philippians 3) in The interpretation of St. Paul’s Epistles to the Galatians, to the Ephesians and to the Philippians (874–875).