1 John 2:1-3, "My little Children, I am writing these things to you that you may not sin. And if anyone sins we have an Advocate with the Father, Jesus Christ the righteous, and He Himself is the propitiation for our sins, and not for ours only, but also for those of the whole world.
And by this we know that we have come to know Him, if we keep His commandments."
I looked up some of the words in the original Greek in which it was written. Here is what I found.
writing: grapho-to describe
Advocate: parakletos-an intercessor, consoler, comforter
righteous: dikaios-innocent, holy, just, meet
propitiation: hilasmos-atonement, payment
keep: tereo-to guard, to fulfill a command
John is describing these things for us, not just giving us a superficial idea of how we can know that we are a Christian. He gives details, word pictures, he spells it out. He doesn't want us to have any doubts.
It is so awesome that Jesus is our Advocate before the Father. He intercedes on our behalf. He is also there to comfort and console us. We all need a comforter from time to time. And He is able to do these things, because He is righteous. He has never sinned, He is holy, innocent of any wrong doing. And because He is holy, innocent He was the only one who could qualify to be our propitiation, the payment for our sins.
Here again we are given a clear picture of how we can know that we are Christians (know God)...we keep, fulfill His commandments. It really isn't a hard concept to understand. Of course, we must ask for strength and desire to do this, as we are unable to do it in our own strength. And God is faithful to give us what we need, if only we would ask.