Catholics believe that Jesus redeemed fallen man by His death on the cross and in so doing became the Mediator between God and man. The grace of redemption was therefore merited by Christ. However, Catholics believe that if these graces are to be applied to man, certain conditions must apply.
Firstly, faith must be present. Secondly, there must exist a genuine sorrow for our sins. Not just the sin of Adam and Eve but our ongoing, daily sins. That sorrow is expressed in works of penance.
In Colossians, Paul says "Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ's afflictions for the sake of his body, that is, the church".
Nothing is actually lacking in Christ's afflictions in one sense. However, Jesus has made suffering redemptive and our sufferings likewise have a redemptive quality when offered to God. This is what Paul is referring to. This is why Jesus tells us to deny ourselves, take up our own crosses and follow Him.
Two facts: Christ suffered the whole pain, paid the whole price, endured the whole penalty for sin; and we live of sufferings and of the Cross, not "despite" Him but "with" Him.
That's why He says, "take up the Cross and follow me," (He never says "Don't take up th cross, that would insult Me: I did it all) --- because we are WITH Him, in His sufferings, death, and resurrection.
Romans 6:4
We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life.
Romans 6:8
Now if we died with Christ, we believe that we will also live with him.
Romans 8:17
Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory.
Romans 13:14
Rather, clothe yourselves with the Lord Jesus Christ, and do not think about how to gratify the desires of the flesh.
1 Corinthians 6:15
Do you not know that your bodies are members of Christ himself?
1 Corinthians 12:12
Just as a body, though one, has many parts, but all its many parts form one body, so it is with Christ.
Galatians 2:20
I have been crucified with Christ and I no longer live, but Christ lives in me. The life I now live in the body, I live by faith in the Son of God, who loved me and gave himself for me.
Galatians 5:24
Those who belong to Christ Jesus have crucified the flesh with its passions and desires.
Hence, penance: not substituing for Christ, but being intimately united with Him.
Please pray for me during this Holy Lent, as I will pray for you!