Perhaps, like me you have wondered about the difference between three Bible words, which I have generally seen as largely synonymous- sin, transgression, and iniquity. I'd like to share a few personal thoughts on this subject. Please note that all Bible comments relate to the KJV.
The word "sin" is very prominent in the Bible and therefore in church theology. Though we often think of sin as disobeying a rule, the Greek word "hamartano" which translates "sin" in the NT means "to miss the mark". The corresponding Hebrew word "chatta'ah" means a "misstep", which sounds much like "missing the mark". The NT view of sin begs the question- What is the mark?
The word transgression in the OT comes from the Hebrew word "pesha" which means rebellion or the guilt or punishment resulting there from. Transgression in the NT comes from the Greek word "parabasis" which means to breach a promulgated law.
The English word "iniquity" in the OT derives from the Hebrew word "avon", meaning the guilt or punishment for that seen as evil. In the NT, " iniquity" translates from the Greek word" anomia" meaning the condition of being without law (lawless) or a violation of the law.
These variations in meaning is particularly interesting when considering I John 3:4 which links both the word "sin" and the word "transgression". That verse reads "Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law". Apparently another way to translate the latter part of this verse would be to say that sin is lawlessness. Now some may think that such is obvious, because lawlessness generally implies acting without regard for the law. However, it occurs to me that the verse is saying that sin is not law or commandment dependent; it exists even without any promoting legal authority. In other words, sin can be practiced even before there is a specific law prohibiting certain actions. This accords well with Paul's thought in Romans 5:12-14, where he says sin existed in the world before the law.
This also makes better sense in terms of differentiating between sin and transgression. Transgression is the violation of a promulgated law, that enacted and enforced by external authority. Sin or "missing the mark" is another matter, indicating that which violates a life principle and misdirects our spiritual evolution. Sin is, therefore, defined by a natural law like that which defines gravity and requires no commandment nor any judicial agent for its enforcement. Again, following Paul in Romans 5 and in Romans 7:13, we see that the law does shine the light of recognition on sin by calling our attention to its many manifestations. This helps explain Paul's other comments in Galatians 3:24 about the laws purpose- to bring mankind unto Christ- and what he ultimately taught about the principle underlying the law (Romans 13:10).
In seeing sin as something other than a legislative violation, we open ourselves to a whole different possibility of understanding the purpose of Jesus' earthly ministry. If sin is not a legal violation requiring restitution in some fashion, then sin mitigation as part of Jesus' mission takes on an entirely new meaning. No longer must God's wrath and pending judgment be the need behind Jesus' sacrifice. Rather than seeking the answer to sin in appeasing or placating the lawgiver deity through some ritual payment, we look for guidance on how to "hit the mark". In other words, the ethical instructions of Jesus become the solution to sin and not his sacrificial death per se. Under this paradigm, his death is a demonstration of what he taught.
The church will insist that the "mark" missed in sinning is God's approval and acceptance, with heaven in the balance. That is certainly a longstanding view of sin and Jesus' purpose. I suggest that there is another understanding of sin which accords much better with the Bible's own insistence that God is Love and that perfect love casts out fear, fear being the inevitable result of an angry God who makes rules and demands compliance under penalty of torment.