I'll get the book down and give you a "greatest hits" cut and put the ISBN - because there's a lot of obscure figures you probably won't know.
As for what I mean by "not existing" is that basically it's because we have much less evidence for them, much further away from their life. What we have of Christ states that not long after his death, he had gained a following of people in the Roman Empire. There's emperors who we have just one passing mention of about 200 years later in a run down of other emperors but nothing else. Yet no one questions these emperors, just Christ - because most people questioning Christ don't understand how ancient sources work.