Q: Please explain the difference between sin, trespass and iniquity.
"When David offered his prayer, he named all three. He said, 'According unto the multitude of thy tender mercies blot out my transgression. Wash me throughly from mine iniquity, and cleanse me from my sin.' (Psalm 51:1,2). Three words; they are all akin but of varying severity.
In Greek, the word for trespass means mistake; we make plenty of those. The one for sin (hamartia) means to 'miss the mark' and iniquity is the worst one. The words to 'miss the mark' suggest shooting an arrow and a bow. This analogy will help you understand it:
If you are totally unskilled and unused to a bow and arrow and you miss the mark, that is trespass. If you have the strength to hit it and you just miss it, that is sin. But if you have the skill and the strength and you deliberately miss it, that is iniquity. Iniquity is sinning against light. It is sinning against knowledge. You disobey God and you know better."
by C. D. Brooks in Questions and Answers.