We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. This is how we know the Spirit of truth from the spirit of deception.
In context
4You, dear children, are from God and have overcome them, because the one who is in you is greater than the one who is in the world. 5They belong to the world; therefore what they say belongs to the world, and the world listens to them. 6We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. This is how we know the Spirit of truth from the spirit of deception. 7Beloved, let us love one another, because love is from God, and everyone who loves has been born of God and knows God. 8Whoever does not love does not know God, because God is love.