In the new testament, I have found three primary definitions for sin that I think are very instructive. These are located in Romans 14:23, James 4:17, and I John 3:4.

1. Sin is doing something despite having qualms about it (Romans 14:23). The classic Biblical example of this is eating meat offered to idols. Paul made it clear in I Corinthians 8 that intrinsically, doing so is not sin. However, he also made it clear that to do so thinking that it may be wrong is sin. Basically, if you have a conviction on something, you need to keep that conviction. Your convictions can and should change as you learn more and transition from a weak believer to a strong believer, but whatever ones you currently hold should not be compromised.
2. Sin is not doing what you know you should be doing (James 4:17). The immediate context of this statement is in connection with saying ‘If the Lord will, we shall do such and such” instead of boastfully presuming on the future. However, I believe it can be taken in a broader sense as well. Now, the verse actually says that if you know what is good and don’t do it, then you have sinned. Does this mean that I must do every good action that pops into my mind? I don’t believe so. This would lead to absurdity. I believe a sensible way to understand this is to do what you should be doing right now. In order to do this, you would either need a paradigm in place to determine a prioritized list of things you should be doing, or else be following any hint of what could be the spirit’s prompting. Your choice would reflect whether you believe reason or instinct should be the guiding force in your decisions.
3. Sin is transgressing the law (I John 3:4). This is where things get controversial. Is John referring to the Torah, the commands of Christ, the instructions in the epistles of Paul, or possibly all three of these? My short answer to this is: I don’t know. However, if you want to know how I currently make sense of this, check out my Scriptural Command Paradigm (also part 2).

Now, I believe most people tend to think of the third definition when they think of sin. They ask, is homosexuality sin? What about gambling? Is it wrong to smoke? And the questions go on and on. What they are basically asking is what does the Bible categorically prohibit? This is a very important, though difficult, question that is worthy of much thought and study.

However, I am here to propose that the first two definitions given above are more fundamental and more important than the specific actions the Bible condemns as sin. One reason for this is that these are not ambiguous like the last definition. You know if you have a qualm about doing something, or if you are not doing what you know you should be doing right now. Even something most Christians think is just obviously sinful, homosexuality, is really dependent on how you interpret Scripture, such as this article and this one show. I have a different understanding of this topic than these authors do, but I am willing to admit that I could be wrong.

This drives sin a lot closer to home. You don’t know what other people’s convictions are usually and neither do you know their priorities, so for the most part judging others is excluded in the first two definitions of sin. The only person you can judge is yourself.

I believe this understanding of sin is very important in witnessing. I know of people who use the 10 commandments to show a person that they are a sinner and in need of forgiveness. While I believe this is done with good intention, I think it is a very poor and inconsistent method of pointing out sin. There are a wide variety of viewpoints on whether the 10 commandments apply to Christians today as well as on how to interpret them.