"Not this time. Pompeii is a fixed point in history. What happens, happens. There is no stopping it." Says who? Says him. "What, and you're in charge?" TARDIS? Time Lord? Yeah. "Donna, Human, no!"
But this is the gift of Pompeii. Every single oracle tells the truth. Even those who don't inhale. You and the Doctor told the people there that you were Spartacus: He was Spartacus, and you were Spartacus. Heroes, interchangeable, self-sacrificing, brave: He was Spartacus. You were Spartacus. They thought it meant you were married. They weren't wrong.
"A name is but a cloud upon a summer wind. But the wind is felt most keenly in the dark. But what is the dark, other than an omen of the sun? I concede that every sun must set..." All true. And the Doctor's answer? "And yet the son of the father must also rise." Every oracle tells the truth.
"A seed may float on the breeze in any direction," said the oracle. And he was talking about you. But nobody should know the story before it's told. (Spoilers!) The Doctor said, "It must be awful being a prophet, waking up every morning: 'Is it raining? Yes. I said so.'" All you need is somebody to tell you to come in out of the rain.
"Do you know I met the Sibyl once? Hell of a woman. Blimey, she could dance the Tarantella! Nice teeth. Truth be told, I think she had a bit of a thing for me. I said it would never last, she said, 'I know.' Well, she would... Let me tell you about the Sibyl, the founder of this religion. She would be ashamed of you. All her wisdom and insight gone sour. Is that how you spread the word? On the blade of a knife?" In the barrel of a gun?
In Pompeii, in Rome and Sumer, anywhere they could still touch God, they celebrated the hieros gamos, the sacred wedding. Take on the garment of a God, channel and bring him into you, and by union with a human you provide fertility: for yourself, for the land and for the people. You save the world. The Sumerian Kings would lie with Inanna's High Priestesses. St. Theresa built a ministry on it. The Wiccans play it out, every spring, with their daggers and their chalices.
But in all this truth, the oracles couldn't see the volcano. The Pyroviles -- that's who was orchestrating it -- had stolen its power, and were ready to build an army. There's nothing the Doctor hates more than armies, so he realized he had a choice. Or more properly, you had a choice: set off the volcano, naturally, invert the system and destroy Pompeii... Or let the world perish in fire. "If Pompeii is destroyed then it's not just history. It's me. I make it happen." He was infecting you, even then, wasn't he? Like huon particles in a coffee cup, making you special without you even knowing it. Taking what was already there, and adding something else. Something he had no right to add.