1 John in particular sets forth some stark oppositions. Here are just a few of them:
If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth (1:6)And the book carries on in the same manner.
And by this we know that we have come to know him, if we keep his commandments. Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, but whoever keeps his word, in him truly the love of God is perfected. By this we may know that we are in him: whoever says he abides in him ought to walk in the same way in which he walked (2:3–6)
Whoever says he is in the light and hates his brother is still in darkness. Whoever loves his brother abides in the light, and in him there is no cause for stumbling. But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes. (2:9–11)
This can sound like our faith is either on or off, light or dark, pure or corrupt. And it can be very troubling because I imagine that almost all believers see their experience of faith as on something like a dimmer switch with varying degrees of light and dark and complexity. We might read 1 John as suggesting that if we see any darkness in us then the light is actually off.
I don't think that would be right. Before most of these challenging texts the letter has already made clear that all followers of Christ sin, that to deny our sin would be a lie, and that God has made provision in Christ to deal with our sin (1:8—2:2).
So what are those dualisms about? Well, I have never studied the book properly so I am not sure. Here is my best primitive guess:
What John is doing is setting forth the fundamental antipathy between light and dark, love and hate, obedience and disobedience, etc. It is not that any person exemplifies all of one or all of the other; every believer is an ever-shifting and complex mix of light and dark. However, to the extent that we are not loving a fellow Christ-follower (say) then to that extent we are not walking with God. John will not allow us to make excuses for ourselves.
Imagine a bottle that can either be filled with air or water. The value of the dualism is not in saying that the bottle is either full of water or full of air. Rather, the value is in pointing out that the air and the water cannot occupy the same space at the same time—they exclude each other. The extent to which you have air is the extent to which you lack water, and vice versa.
Looking at the dualisms in 1 John in this way may help put some of them in perspective and allowing them to function as they were intended—not as a means of making us insecure and depressed but as a challenge to draw closer to God and as a means of pulling the rug from under our excuses.