This is a fundamentally important question in Islam. If the are indeed believers, then Islam is not necessary to avoid eternal torment in hell. However, if the People of the Book are disbelievers, then Allah has a lot to answer for...namely why did he allow for so many people to be led astray by books they themselves did not corrupt? A typical Islamic response would be: "It's a test." But if that's the case, why would any omniscient (all-knowing) being require a test to know the hearts of men? And why create an eternal torture chamber to punish those humans who failed the test when he knew they were going to fail before he even created them? Allah is either a sadist or he is not omniscient.