The answer given is, “Sin is disobeying or not conforming to God’s law in any way.” This is another one where I prefer the older version which answers, “Sin is any want of conformity unto, or transgression of, the law of God.”
There are two categories here, one is passive and one is active.
Passive sin is not doing what we know to be right. Active sin is choosing to do (or desire to do) what we know to be wrong.
It seems to me that even those who would not acknowledge the idea of sin still have feelings that they should be better than they are. Almost any sentence that begins with the words, “I ought . . .” would give evidence in support of this assertion.
The important question is what is the standard for determining right and wrong? The catechism affirms that the correct standard for sin is the law of God. In 2016 this is unpalatable to many people.
But Jesus also affirmed the law of God as the standard when he said, “Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill.” (Matthew 5:17, NASB)
In thinking about our social context, I don’t see that it does a lot of good to go around telling people they are sinners. How many people do you see responding positively to a street preacher that is telling people that they are sinners going to hell? Apart from the grace of Christ, we are all sinners, so while it may be true, it is hardly helpful to do this.
It is really up to the Holy Spirit to convict each of us concerning our own sin. Our job is to point people to Jesus who is the cure.