But on an exegetical level, you must know that is quite an ambiguous, enigmatic verse, which is open to interpretation (an RC on CA answers stated it was “clear as mud").
The Catholic NAB states,
What is lacking: although variously interpreted, this phrase does not imply that Christ’s atoning death on the cross was defective. It may refer to the apocalyptic concept of a quota of “messianic woes” to be endured before the end comes; cf. Mk 13:8, 19–20, 24 and the note on Mt 23:29–32. Others suggest that Paul’s mystical unity with Christ allowed him to call his own sufferings the afflictions of Christ. — http://usccb.org/bible/scripture.cfm?bk=Colossians&ch=1
The conservative Haydoc RC commentary, states,
Ver. 24. And fill up those things....in my flesh for his body, which is the church.[5] Nothing was wanting in the sufferings or merits of Christ, for a sufficient and superabundant redemption of mankind, and therefore he adds, for his body, which is the church, that his sufferings were wanting, and are to be endured by the example of Christ by the faithful, who are members of a crucified head. See St. Chrysostom and St. Augustine. (Witham) --- Wanting. There is no want in the sufferings of Christ himself as head; but many sufferings are still wanting, or are still to come in his body, the Church, and his members, the faithful. (Challoner) --- St. Chrysostom here observes that Jesus Christ loves us so much, that he is not content merely to suffer in his own person, but he wishes also to suffer in his members; and thus we fill up what is wanting of the sufferings of Christ. (St. Chrysostom) — http://haydock1859.tripod.com/id212.html
It sees the application is being what is wanting, and want to apply this sacramentally.
Some Protestant commentators also see as referring to the union the church has with Christ, and suffering with Him as believers are called to do till He returns.
I myself see it as that also, and especially to Paul, as he was foretold what things he himself would have to suffer for Jesus name, (Act 9:18) and while there is no lack in the sufferings of Christ in making the perfect atonement for all our sins to be forgiven, yet practical perfection of the body remains a goal, and as a result of the Atonement believers do instrumentally participate in the perfection of the saints (Eph. 4:12) as we follow Christ, and which entails suffering, both by choice and by consequence, (Phil. 1:29; 2Tim. 3:12; 1Pt. 2:21) in the sense that the body is not one member but many, (1Cor. 12:14ff) and is interdependent (but “co-dependent upon God) and minister grace to each other. Paul thus stated,
And whether we be afflicted, it is for your consolation and salvation, which is effectual in the enduring of the same sufferings which we also suffer: or whether we be comforted, it is for your consolation and salvation. (2Cor. 1:6)
Therefore I endure all things for the elect's sakes, that they may also obtain the salvation which is in Christ Jesus with eternal glory. (2Tim. 2:10
It is also true that we enter in other men's labors, (Jn. 4:38) in the sense that we are affected by the iniquities or virtues of those before us, and Paul expresses debt to the Jews for instance. We in America have entered into the labors of God-fearing men, as well as the shortcomings of others who were not, and today's generation will suffer more from the latter.
And the church, as the interdependent body of Christ, does “dispense” grace as its members act in obedience toward Christ, in sacrificial love. And the more faith-full and virtuous one is, individually and corporately, then the more effectual such can be as an instrument of grace. By application includes everything from prayer to laying on of hands in ministering the Spirit (Gn. 3:5) to posting on FR (sometimes).
Where i see the problem is when the conveyance of grace by born again believers in dependance upon seeing God manifest His power is replaced by formalism, by perfunctory practices and confidence in ritual by any church which fosters such, and make it much like a bank transaction, which may have a form of Godliness but do not manifest the power thereof. And i do come short in the latter, but know that there is a vast difference between the two.
Nobody likes the NAB as far as I know. The friars I hang with use the RSV.
I think the excerpt from the Haydoc is pretty much what I think.
Of course it is unthinkable that there would be anything lacking in the sufferings of Christ. That's what make the verse important. I think it is not limited to his experience but that it pertains to the Church as his fulness, though he would be full without us.
and as a result of the Atonement believers do instrumentally participate in the perfection of the saints (Eph. 4:12) as we follow Christ,
Could you say more about "instrumentally"?
by perfunctory practices
A note: the practices aren't perfunctory. The practitioners sometimes are.
The "risk" of an ex opere operato 'scheme' is precisely that the priest and the people can 'go through the motions.' The benefit is that it doesn't matter -- in two ways. (1) The one person in the congregation who is awake that morning loses nothing if the priest and the other people aren't.
(2)And this is not the only time they have attended Mass. That's not unimportant. Sometimes what looks like a Mass attended or sung asleep is just a coming consolation or perceived grace "building up a head of steam,' so to speak. I never intended to learn the Magnificat, but on a day I really needed to sing to God (in my paddocks where it would scare only the sheep) I found to my surprise that I had the words I needed right there! All those recitations of evening prayer were "paying off."
And finally, as I say about the Rosary, it's risky to judge by external appearances. It just looks like a bunch of people droning on. You can't see from the outside what is going on inside.