Jesus is talking about the time when Jesus is back to the world as King... the second coming.
The time of grace is over by then. That time, whoever is willfully against his kingdom will be punished because they know that Jesus is God and still do not want to acknowledge him.
Jesus is talking about the guardians of faith who rejected Jesus. To prove the point, let's see how another event unfolded. Jesus healed a blind person who was blind from birth. He did not have eyes at all. Jesus made clay out of spit and spread it over his eyes. It resembled the act of creation of the first human, where God made Adam from earth. Elders in their heart... they knew this was God for sure. But they did not want to concede and lose the privileges that they were enjoying. Remember, Jesus did not allow them to do business in the temple.
At the end, Jesus said this remarkable sentence:
John 9:41 -- Jesus said, “If you were blind, you would not be guilty of sin; but now that you claim you can see, your guilt remains.
These guys did know Jesus was God, but did not want to give up their positions. By doing this, they willfully sinned against their conscience. A Muslim, a Hindu or an atheist may not be judged as severely as those Pharisees because they may not be working against their conscience.
James 4:17 -- If anyone, then, knows the good they ought to do and doesn’t do it, it is sin for them.
How much the sin will be, if you know that Jesus is son of God and you don't accept him.