Fellowship With Jesus
A concern of John's first epistle is that we have fellowship with the Father and the Son. He began by stressing the basis upon which we may fellowship with the Father. Walk in the light as He is the light. Confess our sins, don't deny that we have sin. And make use of our "advocate" and "propitiation", Jesus Christ the Righteous.
But what about fellowship with the Son? In 1 John 2:3-6, John now describes how we can "know" that we have fellowship with Jesus Christ. A key phrase in this passage is "by this we know...", found twice. In other words, "here is how we can be sure".