Jesus is talking about the time when Jesus is back to the world as King. The second coming.
Time of grace is over by that time. 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 below event unfolded. Jesus healed a blind person who was blind from birth. He do not have eye at all. Jesus made clay out of spit and spread over eyes. It resembles the act of creation of first human. Where God made body from earth. Elders in there heart they know, this is for sure God. But they do not want to concede and loose the privileges they are enjoying now. Remember Jesus do 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 do know Jesus is God, but do not want to give up there positions. By doing this they willfully sinned against there conscience. A Muslim, a Hindu or an atheist may not be judged as severe as those Pharisees because they may not be working against there 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.