I think that 'sin' is doing something one believes to be wrong, according to the faith/philosophy one claims to live by. Therefore 'sin' can be subjective. It is VERY personal.
For instance; awhile back, Catholics were not allowed to eat meat on Friday, only fish. Even now some Christians make promises for lent; to do, or abstain from doing, something during that time that would ordinarily be fine any other time of the year, or if they had not promised. Well then, for THEM, eating meat on Friday is sinful. If they had promised to, say, abstain from smoking during lent, then smoking would be a sin...as well as being stupid (not the same thing).
In my faith, we are supposed to fast for 24 hours, or two meals, on the first Sunday of the month. So, unless it isn't healthy to do so, if I do not fast, I'm sinning. If you don't believe in fasting once a month, then it's not a sin if you don't.
So 'a sin' is to do, or fail to do, something one believes God expects of you...or something you expect of yourself. So atheists can sin too. All that is required is an expectation of what a 'good person' should do...and a violation of that expectation.
One cannot confuse secular law with 'sin.' It's quite possible to break the law and not sin...for instance, breaking the speed limit to get one's pregnant wife to the hospital might be dumb, but probably not sinful. If one is a pagan islander who figures that killing and eating one's enemies is honorable, and does so in spite of laws against it, it's not sinful...just unlawful. There are many people who have been jailed for things they did not consider to be sins.
And lots of people who go merrily sinning their way through life by only doing legal things.
Anyway, that's my view and I'm sticking to it. The issue then is not to condemn others for being horrible sinners....especially when in their own views and belief systems they aren't, but to see if you can change their minds to agree with you about what 'sin' is. I do not believe that God is going to condemn anybody for committing 'sins' that they don't KNOW are sins.