My dear children, I write this to you so that you will not sin. But if anybody does sin, we have one who speaks to the Father in our defense —Jesus Christ, the Righteous One. 2 He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world.
That second sentence is a mild understatement- "if anyone does sin". That's a given. The good news, and the core of the Good News, is that Jesus has already died for that as the sacrifice to end them all. He's not just our defense attorney, He's taken the rap for us already.
Verse two leans a bit Arminian in that Jesus died for the sins of the whole world; that seems to fly against TULIP's L of Limited Atonement. John's version is only limited by the third rock from the sun; Jesus died for the whole world, and the rest of the universe is on its own.
Not really; John 3:16 would cover any born-again Mars-born kids, since whoever believes in Jesus will not perish, not just Earthlings.
Let's get the tongue out of the cheek and finish-Jesus' got our back. That's comforting.