What Is Still Lacking In Regard to Christ's Afflictions

In Colossians 1:24 Paul writes, "Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church." What does this mean? Of course nothing is lacking as far as Christ's suffering and death on our behalf to redeem us from our sins. Paul is saying that Christ's afflictions continue, even after his ascension, with the persecution of his followers. Paul himself has definitely experienced his share of these afflictions, including being in chains even as he writes.