"Lurking and nourished sins are always a sign..." (D.A. Carson)

From "Basics for Believers" by Don Carson:

"...we are exhorted (in Philippians 4:4) to rejoice in the Lord. The controlling issue is not the style of the rejoicing, but the ground.

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Our delight must be in the Lord himself. That is what enables us to live with joy above our circumstances. As Nehemiah puts it, "The joy of the LORD is your strength" (Neh. 8:10).

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Whatever the mysteries of evil and sorrow, they do have the salutary effect of helping believers to shift the ground of their joy from created things to the Creator, from the temporary to the eternal, from jingoism to Jesus, from consumption to God.

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Obedience to this command is possible because the ground of this rejoicing is changeless. Our circumstances may rightly call from us grief, tears and sorrow. Unless the Lord comes back first, each of us will face death - our own, and if we live long enough, the death of loved ones and friends. And we will weep. But even in our tears, we may rejoice, we will rejoice, we must rejoice, for we rejoice in the Lord. He does not change. And that is why we shall rejoice in the Lord always.

God well knows that a believer who conscientiously obeys this command cannot be a backbiter or a gossip. Such a believer cannot be spiritually proud or filled with conceit, cannot be stingy or prayerless, cannot be a chronic complainer or perpetually bitter. The cure for a crushed and bitter spirit is to see Christ Jesus the Lord and then to rejoice in him. Lurking and nourished sins are always a sign that our vision of Jesus is dim and that our joy in him has evaporated with the morning dew."