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
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.