There are many challenging passages in I John. He saves a real doozy for the end.
That is: “There is a sin unto death” and “there is a sin not unto death”.
It doesn’t really matter where your starting point is, these verses can throw you for a loop.
First, how is there a sin not unto death? Doesn’t Paul and James both say that if you break one precept of the law, that you are guilty of the whole law?
And there is a sin unto death? Only One?
The devil is always in the details, and in this passage, the answer is in the context.