I saw a really good post about this on another forum…I’m not sure if I’m allowed to link to it, but I’m going to quote the response here:
"I have heard this concept before, and I think as stated in the OP it’s a little too simplistic. From what I understand, it is more a matter of how sin affects the soul. Just like committing venial sin predisposes the soul to commit mortal sin, committing mortal sin predisposes the soul to death. It takes an enormous amount of God’s grace to bring us back to life, to contrition, confession, and faith when one sins mortally. Each mortal sin predisposes the soul to lose the faith God’s grace has given us. Eventually, the sinner can tip the balance and lose their faith entirely.
The concept of “fidelity to grace” is tied to this idea. God gives us grace to avoid sinning when we are tempted. If we don’t remain faithful to that grace, we fall into sin. God does reach out to us, but the more accustomed we become to ignoring God’s grace, the less available that grace is to us. Sinning is like spreading soot all over your soul, and when it gets really black and dirty, we can no longer “see” God’s grace.
Have you ever known someone who had a strong faith, completely believed in God and His church etc. Then, almost overnight they lost their faith, then began blasting God and/or the church? While we cannot read people’s hearts, we can be nearly certain that this loss of faith was the result of the person’s stubborn attachment to mortal sin, and their infidelity to grace. Not all hope is lost for such a person, and God will be eager to forgive them if they turn around, but they are less likely to do so of their own volition. It’s very important to pray for people in these situations because they aren’t praying for themselves, and the prayers of other faithful Catholics can indeed save them.
So, it’s not that God won’t forgive sins any longer after a certain number of them, rather that a certain number of sins will lead the sinner to stop seeking forgiveness."