Probably the most quoted passage from the bible. It is sometime called the Bible in a nutshell.
In Context: And just as Moses lifted up the serpent in the desert, so must the Son of Man be lifted up, so that everyone who believes in him may have eternal life. For God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life. For God did not send his Son into the world to condemn the world, but that the world might be saved through him. Whoever believes in him will not be condemned, but whoever does not believe has already been condemned, because he has not believed in the name of the only Son of God. And this is the verdict, that the light came into the world, but people preferred darkness to light, because their works were evil. For everyone who does wicked things hates the light and does not come toward the light, so that his works might not be exposed. But whoever lives the truth comes to the light, so that his works may be clearly seen as done in God.