"I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth.  Who is the liar but the one who denies that Jesus is the Christ?  This is the Antichrist, the one who denies the Father and the Son.  Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also."

Encouragement is often a reinforcement of someone's belief.  This is what John is using his letter for.

John is not trying to convince his audience that Christ is real, or that God loves them, or that fellowship is important.  He is reinforcing that belief in the people of God.  This is how we are meant to use our testimonies as well.

I'm not writing because you need to know the truth - I am writing because you already do.

Sometimes, we need to hear that someone has our backs. 

If you seek righteousness - be reinforced by the truth.