I think a really important question is, “Who is John’s audience in this letter?” John was both attempting to comfort believers with assurance of salvation and to rebuke false teachers.
@Tim_Ramey I think this difficult question of sin and sin centers on one particular belief of the false teachers - namely, that after they had received Jesus they could continue sinning without suffering the consequences of sin.
We see this well using the translation of the following verses from net.bible.org:
1 John 1:6 - If we say we have fellowship with him and yet keep on walking in the darkness, we are lying and not practicing the truth.
I John 1:8 - If we say we do not bear the guilt of sin we are deceiving ourselves and the truth is not in us.
I John 2:4 - The one who says “I have come to know God” and yet does not keep his commandments is a liar
When John says we are liars if we say we do not have sin - he is rebuking false teachers who say we can sin without bearing the guilt of sin. However, he then comforts believers who may fear for their salvation at this point by reassuring them that God forgives those who admit their sin and confess. So John has two audiences in mind at the same time - the false teachers and the true believers (aka little children).
I John 1:9 - But if we confess our sins, he is faithful and righteous, forgiving us our sins and cleansing us from all unrighteousness.
So here we have a picture that is much easier to understand - false teachers saying that they can follow Jesus and still live in sin and true believers trying to find assurance that their sins are forgiven.
Getting back go your question of “Is there a difference between sin and sin?” I think the most difficult verse in I John is I John 5:16.
I John 5:16 - If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that.
What on earth is the “sin that leads unto death” vs “sin that does not lead to death”?
In the commentaries and articles I read briefly, there seems to be agreement on the following facts:
The sin that “leads to death” involves walking away from God and into captivity to sin, satan and the world - in other words, unbelief
The sin that “does not lead to death” is sin committed by a believer who may be disciplined by God for their sin, but is still under His loving care as the Good Shepherd
I think there is good evidence in I John for this claim because the verses right after 5:16 draw a sharp distinction between believers, who God protects, and those under the power of the evil one.
I John 5:18-20 - We know that anyone born of God does not continue to sin; the One who was born of God keeps them safe, and the evil one cannot harm them. We know that we are children of God, and that the whole world is under the control of the evil one.
I think verses 18-20 and verses 16-17 are parallel verses. The “sin that does not lead to death” is committed by those “kept safe by God”. The “sin that leads unto death” is committed by those “under the control of the evil one”.
The following article may prove helpful to you:
So, what is my opinion? Is there a difference between sin and sin?
No. All sin is sin. But sin committed by those under God’s care is repented and forgiven. Sin committed by those sold under the dominion of the evil one is never repented and therefore never forgiven.
As an example - King David committed “sin that does not lead to death” and King Saul committed “sin that leads unto death”. They both sinned. But David remained under God’s care, though God did discipline him for his sin. He admitted his sin was sin and repented.
That is similar to the example you gave of Hezekiah, who repented and turned.