I would take reasoning as my authority. Yes, humanity should help to make good reasoning. I may not be always perfect but I would do my best.
I think the sin word only talks about an imaginary viewpoint. Yes, likely too disingenuous.
I would love to see "In reason we trust" replacing "In God we trust" as the slogan imprinted on currency money.

Sin, as defined in the original translations of the Bible, means "to miss the mark." The mark, in this case, is the standard of perfection established by God. Sin equals breaking the law and I would therefore show that Biblical morality is not up to today's secular standards of morality. I really like using the example of slavery. The Bible never, in either the New or Old Testament condemns slavery. It has all kinds of rules of how to treat your slaves such as if you beat them, make sure they at least live another day. Ex. 21: 20 In my reading of their holy book it contains very little moral or ethical good judgement. You might plant a seed of doubt or you may not, but evidence is on your side.

We face choices ever single day we are on this planet. What would your choices be if survival was on a primative level? I know, I think there is a reality TV show, no TV so no clue. We choose to be good or not. Man created religion and uses it to control, there are some really kind christians out there, from what's been in the media since trump choose to run and then be placed in the New American Daycare Center most of those 'christians' are not nice. I personally do not require a religious authority to tell me where my moral authority comes from. I don't think I was born with sin. In the end trying to convince a true believer is something of a waste.

Save your time next time and turn it around on them. Ask them if they adhere to all of the rules from the bible dictating what sins are? When they finally admit that they don't think wearing clothes of mixed fabrics or eating shellfish or being gay etc. is not a sin, point out that their ability to pick and choose which parts of the bible they adhere to also mean that their moral compass comes from within, not from the book, just like yours.

I don't think sin exists to non-believers?

Openly defy the existence of sin. It is a pattern deliberately reinforced by those who are weak and fearful, as a weapon against those who are strong.

Ok I googled sin and it is used two ways:

Sin-noun-an immoral act considered to be a transgression vs. divine law

sin-verb-commit a sin, commit an offense, commit a crime, break the law, misbehave, go astray.

Seems you could use it as a verb?

A tip for reading and understanding the bible. Sin and Evil share the same two meanings:

1. anything that causes harm
2. anything that pisses off God

The second one can be tricky. It can be for raping or not raping, murdering or not murdering, stealing from those you have murdered or not stealing from those you have murdered, taking sex slaves or not taking sex slaves. What God decides is completely arbitrary and usually revealed after he has killed or not killed someone.
"Praise God!"

If your moral authority believes that pandering to their vanity is more important than preventing rape, you have no moral authority. Sin is therefore meaningless. Worshipping god is in the 10 commandments. Rape is a property crime punishable by marriage.

Instead of sinful I use hypocritical. Sinner = hypocrite. Seems to work for me...