This fearful warning is to be delivered, because there are consequences to unbelief which the unbeliever would like to put out of his mind. Christ is of course speaking of the coming Day of Judgment, though Luke does not use these words explicitly.
There is a trick which the self-righteous play to minimise their sin in their own eyes. It is a dishonest trick and it is exposed by the Lord here. The self-righteous man measures sin by what can be seen externally, and he hides everything else from view. But Christ judges righteously, and he judges according to what a man knows. Increased knowledge means increased responsibility before God. What we do with his message may render us more guilty than those we regard as the worst of men in this world. This is why the children of believers have to be so careful how they listen to the instruction from the word of God.