Question:

If we judge a Jew or a Christian to be a kafir (disbeliever), does it necessarily follow that they will enter Hellfire?

Answer:

Yes, there is no doubt. The Prophet (peace and blessings be upon him) said: “There is no one from my nation who hears of what I have been sent with, and then does not believe in it, but that he will be of the companions of the Fire,” and he explicitly mentioned Jews and Christians “By the One in Whose hand is my soul, no one from this nation hears of me—neither a Jew nor a Christian—and then does not believe in what I have been sent with, but that he will be of the companions of the Fire.”