"Darkness is a harsh term don't you think
Yet it dominates the things I see...
It seems that all my bridges have been burned
But you say 'That's exactly how this grace thing works’"
Mumford & Sons, Roll away your stone
God is light; in him there is no darkness at all.1 John 1:5b
It's common for people to say that they don't believe in "sin/evil/darkness" maybe because, as this song says, those words sound a bit harsh. But its undeniable that there is something wrong in the world. As we look around, darkness is everywhere. Darkness is simply an absense of light, and the darkness we see is the absense of God - the tendancy of every person is to willingly shut out the source of light, to stubbornly resist and choose to stay in the darkness.
"Light has come into the world, but men loved darkness instead of light because their deeds were evil. Everyone who does evil hates the light, and will not come into the light for fear that his deeds will be exposed." John 3:19-20As it says in the book of 1 Peter "... [Jesus] called you out of darkness into his wonderful light." (1 Peter 2:9). We don't deserve this, we've done nothing to earn this, we burned our bridges with God when we decided that we wanted to rule over our own lives. This is grace. We have nothing to offer but He offers us everything. Jesus replaces the darkness with light.