Unbelief is a moral issue because all humans know that the true God exists and deserves their worship so to deny it is being dishonest. Though of course years of denying Him can push this knowledge into the subconscious so that they can actually convince themselves that they do not believe he exists. Also the Nazis belief that allowing jews to live is immoral is just a subjective belief based on their own hatreds while the ancient hebrew belief that unbelief is immoral is based on the objective basis of God's moral character.