This is quite a controversial statement of Paul’s to the Colossians (1:24) we hear proclaimed in today’s first reading (Col 1:24-2:3). I thought I would go to my favorite Bible commentaries to get clarification. I share them with you.
My first go to commentary:
what is lacking: I.e., the suffering that remains for believers in the trials of life. Suffering is a mission for all the faithful as a means of conforming ourselves to Christ (Rom 8:17; Phil 3:10), but suffering is a special calling for ministers of the gospel like Paul, who endure many afflictions in the effort to bring salvation to others (2 Cor 1:6; 4:11–15) (CCC 307, 618, 1508). ● These words could be misunderstood to mean that the suffering of Christ was not sufficient for redemption and that the suffering of the saints must be added to complete it. This, however, would be heretical. Christ and the Church are one mystical person, and while the merits of Christ, the head, are infinite, the saints acquire merit in a limited degree. What is “lacking”, then, pertains to the afflictions of the entire Church, to which Paul adds his own amount (St. Thomas Aquinas, Commentary on Colossians 1, 6).
The Ignatius Catholic Study Bible: The New Testament. (2010). (p. 366). San Francisco: Ignatius Press. Emphasis added.
Next in line commentary in my library:
The most common exploitation of this statement is summarized by St. Alphonsus as follows: […] for the merits of the Passion to be applied to us, according to St. Thomas (Summa theolougiae, III, q. 49, a. 3), we need to cooperate (subjective redemption) by patiently bearing the trials God sends us, so as to become like our head, Christ” (St. Alphonsus, Thoughts on the Passion, 10).
Faith in the fact that we are sharing in the sufferings of Christ, John Paul II says, gives a person “the certainty that in the spiritual dimension of the work of Redemption he is serving, like Christ, the salvation of his brothers and sisters. Therefore he is carrying out an irreplaceable service. In the Body of Christ, which is ceaselessly born of the Cross of the Redeemer, it is precisely suffering permeated by the spirit of Christ’s sacrifice that is the irreplaceable mediator and author of the good things which are indispensable for the world’s salvation. It is suffering, more than anything else, which clears the way for the grace which transforms human souls. Suffering, more than anything else, makes present in the history of humanity the force of the Redemption” (Salvifici doloris, 27).
The Navarre Bible: Captivity Epistles. (1992). (pp. 171-172). Dublin: Four Courts Press.
Catholics are very fortunate to have such a well-developed “theology of suffering.” Many Protestants (and, of course, the world at large) struggle with the understanding of the entire concept. Christ’s redemptive work did not take suffering out of the world. It did make it meaningful, though. We are called to bear it as Our Lord did and St. Paul did.