Dr. R.G. Lee's list of conversion excuses and questions

29. Excuse

 "What is sin."

Answer: Sin is breaking God's law. 

1 John 3:10 

In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.

Everything that is not morally right is sin.

I John 5:17

All unrighteousness is sin: and there is a sin not unto death.

Unbelief is sin.

John 16:8-9 

And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
Of sin, because they believe not on me;

Questionable indulgences are sin.

Romans 14:23 

And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.

Missing the mark is sin.  

Romans 3:23 

For all have sinned, and come short of the glory of God;

Undone duty is sin.

James 4:17 

Therefore to him that knoweth to do good, and doeth it not, to him it is sin.