In Colossians 1:24, St Paul writes: “Now I rejoice in my sufferings for your sake and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church…” [From the Catholic edition of the Revised Standard Version as published by Ignatius.]
I suspect, partly on the basis of my own few experiences, that most modern efforts to understand these difficult words of St Paul are within the modern way of thought which sees salvation as individual rather than as communal. The Bible presents matters differently. For Jews, salvation comes to those who are members of the People of Israel and for Christians, salvation comes to those who are members of the Church. Modern Christians, most certainly those who are Americans, both Catholic and Protestants, have fallen strongly under the influence of a radical individualism even to the extent that some Catholics and many Protestants think it possible to be a Christian and a libertarian—I admit freely that there is a proper part of Christian political thought which could be labeled libertarian but the entire way of thought called `libertarianism’ assumes a view of human nature, and of the nature of created being, in opposition to Biblical teachings that man is both fully individual and also fully communal.
We continue to exist as individuals in these communities of salvation, but we take on the communal being of the People of Israel or the Body of Christ, a term I prefer to `Church’ because I think the Church in the sense of an ecclesiastical organization is but one organ of that Body though the most important organ in some ways.
We Christians can understand this in terms of Trinitarian theology. As Father and Son and Holy Spirit remain individual Persons even as They are one God, so we remain individual human beings even as we come together to form one Body of Christ in which we are united with the Christ as the perfect and complete man. Each human being who goes on to share the life of God is the Body of Christ in a true and absolute sense just as the Father is God in a true and absolute sense and so are the Son and the Holy Spirit.
See What is the Role of the Christian Church in the Public Square? for my early views on the relationship between the Church and the entirety of the Body of Christ. I’ve continued to explore this topic of the true nature of the Body of Christ and will continue this exploration for a while.
What Christ was lacking and is lacking, in His suffering and in other ways, is His brothers and sisters. Christ was complete in His own individual human nature allied with His divine nature and divine Person, but He became one of us and took on our needs and desires. Without us, He is somewhat like a human being with some of his limbs torn off, bloody and mutilated; less graphically, He is incomplete in His communal human nature.
The Son of God chose to be dependent upon other human beings, chose to take up a human nature incomplete in its communal aspects during life in this mortal realm. He took upon Himself a human need for human community—for us, at least for many of us.
We perhaps fill up what lacked in Christ’s affliction in various ways, but certainly by becoming members of human communities and, most especially, of the Body of Christ. We become the friends of Christ, members of the greatest of all communities and one in which the Son of God is Himself a member.