What are three essential elements that make a sin mortal?

All sin is mortal (define:mortal - subject to death)

Sin in its very nature is worthy of death. (James 1:15, Romans 5:12, Romans 6:23) Vicarious atonement would render the sin "already paid for"

I John 5:16-17 speaks of sins that are not mortal. The key is I John 5:18, so there is only one essential element that makes sin mortal...

1) The sin is mortal and leads to death if one is not born of God.

Being born of God means that you have been redeemed from the curse of the law.

The unredeemed will die for their sin because they do not have a vicarious substitute who died for their sins.