I've always been intrigued by the letter to the Colossians and the idea of "filling up what is lacking in the afflictions of Christ on behalf of his body, which is the Church" as it comes us in the first reading in the lectionary today.
Certainly there is nothing lacking in the afflictions that Jesus Christ took to himself for the world's salvation; on the contrary, these were and are superabundant in their efficacy. Nor is there any lack of afflictions in this world, in the misery we insist upon for ourselves with our sins and the injustices and violence that plague and hobble human society.
So what is lacking in the afflictions of Christ? What still needs to be filled up? What is lacking is that not all of the afflictions of this world have surrendered to Christ's work of identifying himself with them. We have not yet consented to God's plan to identify himself with our suffering and alienation so as to reveal the path through it to new life. In theological terms, we have not yet consented to the glory of the cross in which the resurrection is revealed.
Suffering is our opportunity to embrace our Marian vocation of becoming those who conceive and bear God's salvation to the world. In ourselves this means surrendering to the Christ who identifies himself with our afflictions so as to show us the path to new life he has blazed from within them. In others this means allowing our hearts to be broken at the suffering of others so that we might learn the compassion that assimilates our own hearts to God.
What is lacking in the afflictions of Christ is our individual and collective realization of the salvation that we already have.