Well, I don't believe in a god, nor do I believe that "sin" is a good way to label and categorize behavior. But, be it as it may...
Well I think there is by definition sin is a transgression of God's will, missing the mark set up by God.
In that case, one can ask, who cares? What does it matter what arbitrary mark this god person sets?
Well, I suppose that one can be concerned that if one defies this god's wishes, then one might be punished by this god. But if that is the concern, then the question takes on a flavor different than what I believe you are trying to express.
Then the phrase
...I still do struggle with sin from time to time...
becomes "I still struggle with obeying the speed limit," or "I still stuggle with not cheating on my income taxes." Even more to the point, in a dictatorship one might say "I still struggle from time to time with opinions contrary to wishes of the state." But is the problem here the opinions, or the unreasonable demands of the state?
Is this all that "sin" is? Just going contrary to the wishes of this god thing? And is the reason this "sin" is bad is because this omnipotent and omniscient god will punish me for "sinning?"
Maybe that's all it is; maybe I should be saying, "Holy shit! I have to struggle with thinking about naked women, because this god person is going to punish me for it!" But if that's all it is, then the idea of "sin" seems to take on a different flavor than what most people think when they discuss morality and ethics.
To count as an atheist, one needn't claim to have proof that there are no gods. One only needs to believe that the evidence on the god question is in a similar state to the evidence on the werewolf question. -- John McCarthy