Jesus is not saying here that the Jews or Gentiles were not guilty of sin until He came, but that the sin of rejecting God as He really is and rejecting His salvation would not have been put against them. They now have no excuse. Knowledge brings responsibility. There are things which are okay for a child to do but which are not acceptable for an adult to do, because the adult should know better. So it was with the Jews after Jesus spoke to them and revealed the Father to them.

Barclay notes that Jesus did two things: He exposed sin, and then He provided the remedy for sin. He did this second thing also in two ways: by making forgiveness of sin possible and by providing the power for man to overcome sin and do what is right. If you become ill and go to a doctor who then diagnoses you and prescribes the cure for you, you have nobody to blame but yourself if you refuse to use that cure and then die. So it was with the Jews.