The best reason not to believe in sin is the love of it. I'm not going to get into the argument that we don't have free-will, it is very technical and complicated but I find it completely illogical and it goes against biblical teaching. Read the Bible and you'll see it doesn't teach pre-destination, but you can cherry pick verses to make it sound like it does. But if sin didn't exist or free-will and everything was predetermined than God would have no reason to punish us and the most wicked would go to heaven.

For the person who thinks every little thing they do is sin, well I used to think the same way, but I learned God looks at your heart. Not if you missed a spot while cleaning the dishes or forgot to wash your hands at work when it's a set rule, 2 things I do sometimes. Tell your Christian friend that she knows what is right and what is wrong, righteousness and sin isn't as complicated as she's making it.

What would I tell the free-will person? Firstly I'd ask why a loving God would send people to hell for no reason if they weren't able to choose good from evil? God calls all men to repent, but he gives us that choice. I would say free-will is something more spiritual (like consciousness) and super natural than anything. That's why we can't explain how free will works. Just like how we can't explain how nothing created nothing (the universe) and if something created us than what created that something. Our brains weren't made to compute certain things, just like if you tried to teach a bear to read and also animals can't comprehend that we're a higher species in my opinion. They might know we are more capable than them but not a very intelligent higher species, again haven't studied this but it's just my opinion. They are lower beings so they don't understand, just like to God we are lower beings so we don't understand the super natural. What knows the things of man besides a man? And what knows the things of God besides God? If we try to understand it or think we understand it we misunderstand it because you can't understand it. Just like how God is 1 in being (substance) and 3 in person (essence). That's my argument to someone who presents me that I can't choose good or evil or says sin doesn't exist. Absolute foolishness and they are calling God a liar.

For the person that thinks everything is a sin I would tell them God looks at their heart, not if they missed a spot. I'd really work on them with overthinking and anxiety because that's exactly what it sounds like. But something you could tell them that might help is to see a therapist about overthinking and anxiety.

(If you want to contact me I'd really like more details about this person who sees every action as sin to see if I can do anything that would help, I don't have enough details to know what would help her).

Thanks for the conversation.