My definition of sin in one sentence: “Any action, or inaction, by a human being that causes harm to a living creature."
That includes harm to another person or to ourselves. It includes abuse of the animal kingdom and even the environment because when we damage God’s creation we cause harm to ourselves and the animals we share this planet with. We know that.
This is my moral code and when I am faced with a moral problem I apply this code. For example, promiscuity causes mental and physical harm in so many cases that to engage in it must be regarded as a sin. Two people, of any persuasion, living together in love, caring for the other and being faithful to each other, causes no harm to anybody, and nothing, not even holy scripture is going to make me think otherwise.
If this definition of sin was my idea, then I could be accused of merely coming up with an arbitary set of rules to suit myself. The very thing that I accuse so many other people of. However, I cannot claim ownership. I got it from that bloke. You know the one. He kept going on about loving God and loving your neighbour as yourself. How they were the only commandments that we needed in life. My pithy little definition of sin is based so obviously on those commandments that I really ought to be paying him royalties.
Take out belief in God and isn't this exactly what Humanists believe. Well, yes it is. God is not necessary for this conclusion. I cannot think of any really material difference in the outlook of mainstream Christians and mainstream Humanists except for the Christian attribution of 'everything' to God, and their adherence to some frankly odd behaviour demanded by those who wrote their Holy book.
My thanks to Mad Priest. The original sermon is at: http://jh002a0382.typepad.com/madpriests_bog_standard_s/