How we cite our quotes: (Book.Chapter.Paragraph)
Quote #4
The crowd answered him, chant and counterchant. Then as the moon lifted clear of the hill, the priest lowered his arms and turned. What he had offered to the goddess, he now offered to the worshippers. The crowd closed in. (II.9.14)
Merlin catches Belasius out on the island of the druids when he witnesses a human sacrifice from afar. At the moment, Belasius is offering the victim up to his goddess—something that Merlin isn't supposed to see. While Merlin isn't really into druidism, he understands that there's a power there. Belasius assures Merlin that he'll pay for his curiosity. We don't see that "curse" play out here, but it's definitely coming in the next books, folks—don't say you weren't warned.
Quote #5
"Uther has no love for them. He is one of Mithras' fanatics, and light is the enemy of darkness, I suppose." (II.12.24)
Ambrosius explains to young Merlin why Uther hates Belasius with a heat of a thousand suns. Bottom line: druidism is an old religion based on darkness and secrecy, while the cult of Mithras worships the light. For Uther, it's like the classic good vs. evil battle. Merlin and Ambrosius don't see things in such black-and-white terms, but they have to acknowledge that others around them see things in such simplistic ways.
Quote #6
[…] I slowly lifted both arms towards the sun in a kind of ritual gesture, such as I had seen priests use in summoning spirits. If I at least made some show as a magician it might keep them at bay, the priests in doubt and the King in hope, till I had the time to remember. (III.10.18)
Merlin's fronting in a big way here, playing on the superstitions and beliefs of Vortigern's men. He knows that they expect him to be supernatural through and through, but Merlin wants to solve Vortigern's tumbling walls problem with memory and engineering. He knows from his past explorations that there's a leaky mineshaft under the tower—but that explanation won't impress the men who want to kill him. Way to play, Merlin.