Who Would Make This Up?

I am reading through the Gospel of Mark during Lent. I was reading Chapter 9 yesterday and some verses caught my attention:

18: “And whenever this spirit seizes him, it throws him violently to the ground. Then he foams at the mouth and grinds his teeth and becomes rigid. So I asked your disciples to cast out the evil spirit, but they couldn’t do it.”

32: They didn’t understand what he was saying, however, and they were afraid to ask him what he meant.

34: But they didn’t answer [Jesus], because they had been arguing about which of them was the greatest.

38: John said to Jesus, “Teacher, we saw someone using your name to cast out demons, but we told him to stop because he wasn’t in our group.”

Skeptics and critics of Christianity may say that the Gospels are fictional. If the Gospel writers were merely inventing stories to try and influence people, why would they make themselves look so bad so often?