What is Lacking in the Sufferings of Christ

Much head-scratching has been provoked by these mysterious words uttered by St. Paul in his Letter to the Colossians: "I rejoice now in the sufferings I bear for your sake; and what is lacking of the sufferings of Christ, I fill up in my flesh for his Body, which is the Church' (Colossians 1, 24).  St. Thomas Aquinas gives a very clear explanation of these words in his commentary on St. Paul's letter:

“ ‘That I may fill up what is lacking in the sufferings of Christ.’  At first glance, these words can have a wicked meaning, namely that the Passion of Christ was not sufficient for redemption, but the sufferings of the saints are added to complete it.  But this is heretical, for the Blood of Christ indeed is sufficient for redemption -- even of many worlds.  1 John 2, 2: ‘He himself is the propitiation for our sins.’  We should understand that Christ and the Church are one person, mystically speaking: the Head of the Church is Christ, whose Body is all the just.  Every just man is a member of this Body. 1 Corinthians 12, 27: Now you are the Body of Christ, member for member.’  God, in his predestination, has ordained that as many merits there should be through the whole Church, so many are in the Head as in the members, just as he has predestined the number of the elect.  Among these merits are, first of all, the sufferings of the saints.  The merits of the Head, Christ, are infinite.  A saint shows merit according to the measure of his sufferings.  And this is why Paul says: ‘I fill up what is lacking in the sufferings of Christ’ -- that is, of the whole Church, whose Head is Christ.  ‘I fill up’, that is, I add the measure of my sufferings.  And he does this, ‘in my flesh’, that is, I myself suffer.  Or, those sufferings which are lacking in my flesh.  This is the ‘lacking’: that just as Christ suffered in his Body, so he suffered in Paul, his member, and so also in others.  Paul does this ‘for his Body”, which is the Church, which was to be redeemed by Christ."