One of the themes of John's writings (not just the gospel) is the contrast of darkness and light. John brings out this theme in chapter 3 after Jesus's discussion with the Pharisee Nicodemus. (As a note, I agree with the majority of scholars who think Jesus's words end in verse 15. Greek didn't have quotation marks, so we aren't positive.)
God sent his Son into the world to save it out of darkness. Most people, however, loved the darkness and rejected the light that the Son provided. They preferred their wicked deeds over the obedience and pure heart that would please God. This is a theme one finds in John's writings. I think it is also worth pondering.