[Campion]

You say:

If you believe you must add anything to what Christ did

Paul says:

Now I rejoice in what I am suffering for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church. -- Colossians 1:24

[MHGinTN]

Two little pints:

1) the Church Paul is referring to is not the roman catholic church;

2) Paul is speaking of the suffering in the body, the physical life being lived, not of souls in some mythical RCC invention called purgatory.