The six-word money quote is bolded below:

In spite of persecution and poverty, they experienced an abundance of joy, which resulted in a wealth of generosity (the Greek uses cognates, “the abundance of their joy abounded . . .”). In the New Testament the Christian’s experience of joy has no correlation to his or her outward circumstances. Paradoxically, Christians can experience joy in the midst of great persecution and personal suffering. Poverty overflowing into wealth also may seem paradoxical, but it fits the crazy-quilt logic of the gospel: joy + severe affliction + poverty = wealth. Here, wealth relates to a wealth of generosity and joy multiplied. Material wealth, on the other hand, may cloak spiritual poverty, as Christ’s condemnation of the wealthy but tepid church at Laodicea reveals (Rev 3:14–22). That church considered itself rich and prospering, but the Lord considered it “wretched, pitiful, poor, blind and naked.” By contrast, Christ praises the poverty stricken church at Smyrna, also beset by affliction, as rich (Rev 2:8–11).

—David E. Garland, 2 Corinthians (New American Commentary 29; Nashville: Broadman & Holman, 1999), 367.