And this is the message we heard from him, which we forward to you: God is light, and there's not one bit of darkness in him. If we say that we have fellowship with him and we live in darkness, we lie and don't live out the truth. . . . The one who claims to be in the light and hates his brother is actually in the darkness (1 John 1.5-6, 2.9).
Here John tells us a bit about the concrete effects fellowship with the Father and the Son (cf. 1.4) should have on a person. The "light" of God should be communicated to the person who lives in fellowship with him; if there is darkness, there is no fellowship with the light. To borrow a theme from a previous post on Stăniloae and deification, fellowship with the Holy Trinity should confer upon the human persons the characteristics and attributes of the divinity which lead to eternal life (cf. 1.2).
What sorts of things must we get from God, then, if we are in fellowship? John emphasizes here love: if someone claims to know God but hates his brother, then that person is a liar and in the darkness. As he says later: Whoever doesn't love, doesn't know God because God is love (4.8). We must become loving when we live in fellowship with God, because God is love and he confers his love to us; it is a part of God's light that he loves, and when this light casts out our darkness, it leaves in its place a vibrant love.
It is important here to note that, among other things, John envisions a concrete change in the life of the person who comes into fellowship with God. If you claim to know God, if you say that you have met the Holy Trinity, then there ought to be something different about you.
This is a tricky issue, since many of us know that change is a slow and arduous process. Oftentimes we may feel that some vices are "here to stay," so to speak, or at least for a long time. Certainly we can see others whose habits seem slow to change, especially if their habits are abrasive and bothersome to us. Interestingly, however, John envisions one concrete form of change as particularly important: love for our brothers and sisters. If we've come to know God, then we will love those around us, faults and vices and all.