‘And this is the judgment, that the light has come into the world, but men loved darkness rather than light because their deeds were evil'.
God bases His condemnation on the fact that Jesus has come as ‘the light' into the world (John 1:4-5; John 1:9; John 8:12), and by His life and teaching has offered the light of life and revealed the light of truth. But men turn from Him because they love their sins and His light therefore shines on them and condemns them. They do not want to give up their lives which ‘come short of the glory of God' (Romans 3:23), the glory revealed by Jesus, and so they reject Jesus and even say evil things against Him, and thus are in danger of the unforgivable sin, final rejection of the clear testimony of the Spirit (Mark 3:22 with 28, 29). If we refuse to open our lives to the light of Jesus we have no one to blame but ourselves when we are finally condemned.
When we pick up a rock in the garden the light shines where it was previously dark and we find there many unpleasant creatures that immediately scuttle for cover. So when Christ's light shines on men they too will respond or run for cover, depending on the state of their hearts, and the result of what they do will determine their eternal future.
‘The only begotten Son of God.' Strictly speaking in earthly terms Jesus was not ‘begotten of God' for on earth to be begotten is to come into existence after the begetter. The idea is rather that He is true God and of the same nature as the Father. Unlike all others He is not a created being.