Sorry, honey, but John is not contradicting himself in the same letter. He didn't say that 1 John 1:8 is a Christian. Shocker!
I hope you enjoyed your popcorn, but you missed some. What, no emoji for a potty break? LOL You didn't "add." I'm not on your road. Where did you go. I thought we were on the same road.
Believing John is describing all levels of Christians, even the highest who never stumble in 1 John 1:8 is the key mistake the western Church makes in their understanding of an eastern writing style, so they cannot believe 1 John 3:9 as stated. They must change the second to keep their warped understanding of the first.
The Church also rolls over the first sentence of 1 John 2:1 to get to the second, as if it were a lie. "My little children, these things I write to you, so that you may not sin." Period. Believing 1 John 1:8 in the way the western Church does currently, makes this sentence obsolete. A lie in itself.
Jesus is the Advocate to the Father for the whole world, not just baby Christians still immature in some of the fruit of the Spirit. "And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous. 2 And He Himself is the propitiation for our sins, and not for ours only, but also for the whole world.
1 John 1:7 shows that while walking in the Spirit, the immature fruit of the Spirit keeps getting cleansed as we grow and all other slight mishaps that we don't even know we are doing. They are unintentional because of the Spirit within. What it is not saying is that Christ automatically cleanses us of lawless willful sins unto death as the Reformation taught and OSAS began. Luther even wrote a letter saying SIN BOLDLY. Exactly what Paul spoke against in Romans 6:1 "What shall we say then? Shall we continue in sin that grace may abound?"
What John is NOT saying anywhere is "believers may commit sin, but they cannot practice sin as a settled way of life." No where does John give permission to willfully sin. Just the opposite.
I'm not sure who invented the term "sinless perfection," do you? I've never heard anyone teach that doctrine as such. But I've heard plenty of pastors on the radio speak against it. That term consists of two words that are a lifetime apart, but both possible."
Sinless is righteous, having no desire to break God's laws. Again, I repeat, that is what Jesus as the Author of our faith begins our walk with Him. That is freedom from lawlessness, John also calls sins unto death.
And perfection is holy as Peter stated where we come to the place of no immaturity also in the fruit of the Spirit and "never stumble," even any sins not unto death, both found in 1 John 5:16-17. This concept has not been upheld in the church since it was taken over by Rome and the dark ages began. And didn't get repaired during the Reformation; in fact, they brought back to life a whole new meaning of grace and made it again a license to sin, unmerited favor. Jude 1:4.