It would be easy to read the Gospel of John today, and say, "Hey, Jesus doesn't condemn anybody - it says so right here: I do not condemn him! It's okay to ignore Jesus' words - He's cool with that." But -like everything in the Bible- you have to read the whole passage, get the context: "Whoever rejects me and does not accept my words has something to judge him: the word that I spoke...."
The Bible contains around 100 references to light - more than a dozen in the Gospel of John. In that Gospel, Christ identifies Himself as the Light, the Light that brings truth to a dark world.
The funny thing about being in the dark is that we adjust to it. It starts to seem normal. You know, it's like when you lose electricity and you get out the candles and the flashlights. You manage, you do what you need to do in the tiny flickers from the wick and the battery. Then, when the lights come back on, you are nearly blinded - it's harsh, almost too much.
We get used to being in sin. We adjust. What first bothered us, then pricked our conscience, no longer seems to be a problem at all. We're used to it. When a light gets shined on it - a word from a sermon, a mention from a loved one concerned for us - we shy away; the light is too much. But we need that light.
You don't want to just "manage" in your life. You don't want to accommodate the darkness, and fumble around. You want to have light - even if it seems harsh at first - so that you can see everything that you need to see. We are not shooting for dimness, we are aiming for brightness and light and truth and illumination. And that is what Christ promises in the fullness of His word.