I think you are on the right track. How can we add to what Christ has perfectly done? Can our works even as believers compare to what Christ has already accomplished? Please read what Paul had to say to those who only wanted to add circumcision to Christ’s finished work. Circumcision by itself, for a Jewish believer might be considered a good work!
It is not so much that we can “add”, but we can participate, and be included in what He has perfectly done.
" 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…" Col 1:24
Now would Paul, of all persons, ever imply that anything was “lacking” in the sufferings of Christ? Surely not! Yet, there are experiences that we have as human beings that Christ did not have. Paul lists a few of his own:
2 Cor 11:23-29
I am talking like a madman - with far greater labors, far more imprisonments, with countless beatings, and often near death. 24 Five times I have received at the hands of the Jews the forty lashes less one. 25 Three times I have been beaten with rods; once I was stoned. Three times I have been shipwrecked; a night and a day I have been adrift at sea; 26 on frequent journeys, in danger from rivers, danger from robbers, danger from my own people, danger from Gentiles, danger in the city, danger in the wilderness, danger at sea, danger from false brethren; 27 in toil and hardship, through many a sleepless night, in hunger and thirst, often without food, in cold and exposure. 28 And, apart from other things, there is the daily pressure upon me of my anxiety for all the churches. 29 Who is weak, and I am not weak? Who is made to fall, and I am not indignant? "
Paul had many experiences that Jesus did not, just as Jesus promised him:
“I will show him how much he must suffer for the sake of my name.” Acts 9:16
Did Paul think that his suffering could “add” anything to Christ? No, it is clear from his writings that he understood the suffering of Christ to be complete. What, then, does the Apostle mean when he says “make up what is lacking”?
Each of us has a unique human experience, and as we take up our cross and carry it, in the grace of God we are able to join our sufferings with His, for the benefit of the Body of Christ. This is part of our spiritual work, our worship.
“I appeal to you therefore, brethren, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. 2 Do not be conformed to this world but be transformed by the renewal of your mind, that you may prove what is the will of God, what is good and acceptable and perfect.” Rom 12:1-2
It is our part to “prove” by presenting our bodies as a living sacrifice.
We must also be carefull to clearly define terms. Faith to a Catholic is not the same as Faith to an evangelical. For a Catholic faith is usually mental assent to doctrine and can also be the summation of the belief system, like what does your faith teach, similar to saying what does your denomination teach. Faith for the evangelical is usually thought of as heartfelt Faith (In Christ) that results in regeneration.
This is a very prejudicial statement, and sad to read, but not surprising coming from someone who never understood Catholicism. I agree with you, Catholic Faith is that faith taught by Jesus, and reflected in the NT. Catholic faith is that in which we offer our bodies as a living sacrifice, and one in which we join our sufferings with that of Christ, to make up what is lacking.