This may or may not go anywhere: Papist is not wrong. He simply began in the middle of things and is using technical terms that are not self-explanatory.
Try starting with justification, which is the cleansing of sins and the restoration of righteousness.
Well...that righteousness is never our own: righteousness [holiness] belongs to God and through his grace we are restored to a state of sanctity each time we are absolved of our sins, where we remain until we sin again.
We can not even be open to receive grace that will put us into a state of sanctity in this life until we've been restored to the state of original justice that belonged to Adam and Eve [through Baptism into Christ]. That Baptism and Communion with Christ is made possible by his life, passion, death, resurrection,and ascension into heaven. That is, essentially, the redemptive act of Jesus Christ.
That redemptive act and the ongoing work of salvation is what the Catholic Church means when it refers to the merits of Christ.