EWTN Catholic Q&A
offer it up
Question from Kathleen Weller on 02-27-2017:

Although I've explained the Catholic practice of "offer it up" and combining our suffering with that of Jesus', one of my protestant friends has challenged me to find biblical support for the practice. I have found it in the Catechism but have not located a biblical reference. Can you help us with this? Thank you.

Answer by David Gregson on 11-09-2017:

The classic reference is Colossians 1:24. "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..."

This means not that the sufferings of Christ were in any way deficient, but that He permits us to unite our sufferings with His as an offering for the good of others.

The difficulty in helping your Protestant friends understand is that they assume only what is in the Bible has divine authority, whereas Catholics believe that the Church has been given authority to interpret Scripture in light of the Tradition handed down from the Apostles.