Now one could spend a lot of time showing why this is wrong biblically. The man speaking with him tried to do this for a while until the Catholic stated that Christ gave the Roman Catholic Church the authority to interpret Scripture to him, he could on his own. At that point there is no reason to reason from Scriptures because it is not the final authority.
But one thought that came to me that I would liked to have asked him goes back to mortal and venial sins. The whole division of sins into "grave" sins and sins of "lesser" matters is without biblical warrant. Certainly some sins affect other people and civilly stealing is worse than murder but the issue is whether God looks at some sins as worthy of his wrath in Hell while others he is mostly just annoyed at and will allow them to be paid for in purgatory or a few hail Marys, etc. This is what I found so troubling with what this man (read the Catholic Church) teaches.
The more serious error of dividing sins this way is that it suggests that we don't need to be justified for some sins. Only the worst of sins remove us from grace so necessarily the lesser ones don't offend the Holy God too much so a slap on the hand will do. But in fact the least sin is an unforgivable trespass against God. To spend a few minutes living without a perfect desire to honor the Lord is a sin worthy of Hell. In other words, there are no "slight" (to quote one Catholic theological work) sins. Such a definition of sin makes the holiness of God to be much less than it is as if there are some reasons to sin against God that aren't as bad as others.
I kept thinking where do you or your church get off thinking some sins damn you while other sins will just cause you some irritation? Of course as one studies the history of the Catholic Church we soon realize that the elaborate scheme of sins and the always possible chance to fall from grace gives the Roman church countless ways to hold Hell over people even if they are supposedly saved. This is why when a Catholic priest is ordained he becomes "another Christ". Once sinful men have been given the authority to save or damn others it didn't take long for them to devise a system to extract money and service from them or send them to Hell.
How much better to see the full atonement made by Christ who died and covered all sins. We cannot fall from grace because it simply means a gift, not a wage. Something offered only on the contingency that one fulfills his end of the bargain is not a gift it is a merit or a wage. If God keeps giving and taking justification from us it is not a gift. And to redefine grace by saying that is gracious that God gives us the opportunity to work for justification doesn't help the matter either.
Rom 4:7 "Blessed are those whose lawless deeds are forgiven, and whose sins are covered; Rom 4:8 blessed is the man against whom the Lord will not count his sin." If we are constantly committing mortal sins and the Lord is constantly holding them against us who exactly is the blessed man. Certainly no one on this earth. Praise God I am safe in the Sinless One who always did what pleased the Father without fail and his righteousness has been given to all who trust in Him.