Col 1:24 as in the previous post - that is a good start.
*]How to explain this to an atheist who doesn’t believe in scripture anyway.
It’s a question of Justice. An atheist might understand that.
Sin takes away good from the world. Every sin an atheist commits, even those little jabs in internet discussions - those are an injustice to others and to the world.
Who is going to pay back that injustice? How do you pay back the pain caused by sin?
Well, the way it works is - the pain caused by sin, can be repaid back by the pain of reparation or penance.
Some people sacrifice themselves to pay back sins they didn’t even commit. This is a high degree of charity. Atheists generally don’t pay back their sins - so the injustice in the world increases. If nobody pays back any pain to make reparation - after a while, the balance gets skewed and only God can bring back the balance. This is what happens when we see various disasters of various kinds. If more people had made reparation for sin, there woudln’t be those disasters.
That’s why, as the world turns farther from God - there will be more tragedies, crises and disasters of all kinds - because nobody is paying back what they owe due to an increase in sin (think global warming, terrorism).
Reparation is what saves us from much evil.
*]How to offer up your pain in practise
A classic thing we were taught in the old days - a simple phrase: “Offer it up!”
We even had our football coach yelling that to us (Catholc school). A guy would be all upset for dropping the ball or something like that – the coach would yell “Offer it up!”-- he meant, take that suffering and offer it to God.
How to do it?
it’s really just as simple as a prayer “Dear Lord, I offer you this pain I’m suffering right now, in reparation for my sins and for the salvation of souls most in need”.
That is probably the most powerful prayer you can offer.
*]And then how to actually pray to achieve this.
The prayer “Thy will, not mine, be done” is what we’re saying.
No matter what happens in our day - once we start feeling pain (frustration, upset, irritation, bumps and bruises, fears, humiliations … whatever - we make that prayer “Dear Lord, I offer you this pain … please accept it, along with the Offering of Jesus on the Cross in the Holy Sacrifice of the Mass”.
The story of Fatima has a lot on this. God is very pleased with that offering and it is the most powerful way to obtain great graces for people and ourselves and the world.