Rejoice in Suffering
The language is reminiscent of 1:18, where “church” refers not to a local assembly of believers (as is usually the case in the New Testament) but to the “universal church.” By referring to the church as Christ’s body, Paul highlights the corporate solidarity that Christ’s people enjoy with him
So we hesitantly adopt the sense “I am filling up in order to complete” what is lacking in regard to Christ’s afflictions.
It is not that there is anything lacking “in” the atoning suffering of Christ but that there is something lacking “in regard to” (TNIV) the tribulations that pertain to Christ as the Messiah as he is proclaimed in the world
The difference may even be suggested in the vocabulary that Paul uses, since he shifts from “sufferings” (Gk. pathēma) to “afflictions” (Gk. thlipsis), this latter word never being used in the New Testament for Christ’s redemptive sufferings.
What is lacking, then, needing to be “filled up,” are the tribulations that are inevitable and necessary as God’s kingdom faces the opposition of the “dominion of darkness” (cf. v. 13). As members of Christ’s own body, his people participate in the sufferings of Christ himself.
It is in this way that Paul’s sufferings are “on behalf of” the church, including the Colossian Christians. And, of course, as a prisoner for the gospel, Paul is suffering for them even as he writes. As members of the fellowship of those raised with Christ and forming therefore part of Christ’s body, we also are the beneficiaries of Paul’s suffering.