These four verses could be almost all a Christian needs to know to be a complete, informed follower of Christ. It provides a description of the saved Christian: 1) How we can know we are His. 2) How we can tell if we are lying about it or tell if the truth is not in us. 3) The means (to again) know we are in Him. 4) What this ought to actually look like in the life of the true believer.
We often believe what we have been taught and what works for us and we are frequently resistent to altering our beliefs and doubts because to do so causes discomfort, upheavel, and a disconnect from what we have long relied upon to give us meaning, peace and hope. For most human beings, knowing and knowledge is incomplete and based on information and experiences that are limited in scope and depth. Truth however is what it is. It is not relative. It is not uncertain. It is set in concrete. Truth is His and He is not only the author of it, He is Truth itself. To know Him is to know Truth (John 17:3).
Why is knowing Him important? It means we are relating to Him, we are understanding Him, we are involved with Him - the living God. If a person finds themselves believing on Him and loving others, they will know that He is in them and that they are in Him. Those who keep His commandments, in him is the love of God perfected.