I set out to answer this question for myself personally and very quickly came to realize that this question is not as easily answered as I had hoped. I had hoped to be able to define it in a way that I could simply provide a verse “as written in Hezekiah 4:13, sin is …” I am not sure how much reading you have done lately in the book of Hezekiah, but it does not define sin anywhere in there. (For any that do not know, Hezekiah is not actually a book in the bible, and is commonly used to describe things that many incorrectly believe are written in scripture. I myself have been known to “quote” from the book of Hezekiah, usually in an attempt to justify something to myself.)
That is not to say that there is no definition of sin. Google has one, sin is “an immoral act considered to be a transgression against divine law.” I like that definition. The only problem with it is that the definition works only because it is appropriately vague.
I have struggled some to try and discern if there are particular actions in my own life that are in fact sinful. C.S. Lewis has a great quote from Mere Christianity about sin, but in the quote he describes how someone in the middle of sin is not able to discern their own sinfulness. He says “When a man is getting better he understands more and more clearly the evil that is still left in him. When a man is getting worse he understands his own badness less and less.” When asked what the definition of sin is, Billy Graham responded “a sin is any thought or action that falls short of God’s will.” That would be an excellent definition, if only we are batting 1.000 when it comes to discerning God’s will.
There is not only the problem of being able to define sin for ourselves, because we could all agree that we would like to avoid sin as much as our earthly nature would allow, but this also matters in the way others view us. As evangelist for Christ it is important to be cognizant of the way others view us. I am not saying that outside perspective should rule over us, but we should be aware if our actions are bringing glory or aversion to our Savior.
There are a couple of approaches that I think that you could take. The 10 commandments is a good place to start. There is also the Old Testament law, but then there is Romans 7:6 “But now we are released from the law, having died to that which held us captive, so that we serve in the new way of the Spirit and not in the old way of the written code.” Without the OT law, that means that there is no list! To my accounting brain, to not have something like the OT law is akin to the IRS getting rid of the tax code, but still expecting everyone to pay their share. That is crazy talk!
But, that is exactly what Christ did. Christ said that we do not need a list, a code, or a rule book, all we need to do is “Love the Lord your God with all your heart and with all your soul and with all your mind. This is the first and greatest commandment. And the second is like it: Love your neighbor as yourself. All the Law and the Prophets hang on these two commandments.”
I think that to most of us, very much so including myself, this makes us uncomfortable because it does not answer questions like “what is and is not a curse word?” or “what movies are ok to watch?” or “is speeding a sin if I do not get pulled over?” I believe that our heavenly Father knows us, and knows that if we had a rule book we would rely solely upon ourselves and our rules, and less upon Him. It would do us all well to love our God and our Neighbors more, worry about our rules less, and rely on the Holy Spirit to guide us in defining sin in our own lives.