Sienkiewicz was a Catholic, which is rather evident in his portrayal of Peter, especially. The book is, of course, strongly Christian, but does not make the mistake of portraying the Christian characters as flawless individuals. They have faults and failings: they falter and doubt, make mistakes, and fall back into their old ways on occasion. Some, like Crispus, misinterpret scripture and must be set right. In short, they are normal people. This is what makes it so affecting when Nero's persecution starts: these ordinary, fallible people have found something which is worth dying for. And not just dying courageously- many can do that- but to do so with assurance of something much greater to come. This is why they refuse to take steps which would save them from death (denying Christ), and also die forgiving those who have done this to them... an idea which would be completely alien to the Romans.