“This type of construction is called opus reticulatum,” said Lily, pointing to the walls. “It’s masonry work that alternates bricks with tufa.”

“Two-fer?” It was the American man again. The stupid questions were always his. “Is that, like, stronger than one-fer?” Only his wife laughed, a high, annoying whinny.

“Tufa,” said the Englishman, “is actually compacted volcanic ash.”

“Yes, that’s exactly what it is,” said Lily. “It was used quite often as a building block in Roman homes.”

“How come we never heard of this tufa stuff before?” the American woman asked her husband, implying that, since they did not know about it, it could not possibly exist.

Even in the gloom, Lily could see the Englishman’s eyes roll upward. She responded with an amused shrug.

“You’re American, right?” the woman asked Lily. “Miss?”

Lily paused. She did not like this personal question. “Actually,” she lied, “I’m Canadian.”

“Did you know what tufa was before you became a guide? Or is that, like, just a European word?”

“Many Americans aren’t familiar with the word,” Lily said.

“Well okay, then. It’s just a European thing,” the woman said, satisfied. If Americans didn’t know it, it couldn’t possibly be important.

“What you’re seeing here,” said Lily, quickly moving on with the tour, “is what’s left of the villa of Titus Flavius Clemens. In the first century A.D., this was a secret meeting place for Christians, before they were openly accepted. It was still an early cult then, just gaining popularity among the wives of noblemen.” She turned on her flashlight again, using the beam to direct their attention. “Now, we’re moving into the most interesting section of these ruins. This part was uncovered only in 1870. Here we’ll see a secret temple for pagan rituals.”

They crossed the passageway, and Corinthian columns loomed ahead in the shadows. It was the temple antechamber, lined with stone benches, decorated with ancient frescoes and stucco. They wandered deeper into the sanctuary, past two shadowy niches, the site of initiation rites. In the world above, the passing centuries had altered streets and skylines, but in this ancient grotto, time had frozen. Here, still, was the carving of the god Mithras slaying the bull. Here, still, the gentle rush of water whispered from the shadows.

“When Christ was born,” said Lily, “the cult of Mithras was already ancient; he was worshipped for centuries by the Persians. Now, let’s consider the life story of Mithras, what the Persians believed about him. He was God’s messenger of truth. He was born in a cave at the winter solstice. His mother, Anahita, was a virgin, and his birth was attended by shepherds bearing gifts. He had twelve disciples who accompanied him as he traveled. He was buried in a tomb, and later rose from the dead. And every year, his rising is celebrated as a rebirth.” She paused for dramatic effect, looking around at their faces. “Does any of this sound familiar?”

“That’s Christian gospel,” said the American woman.

“Yet centuries before Christ, this was already part of Persian lore.”

“I’ve never heard of this.” The tourist looked at her husband. “Have you?”

“Nope.”

“Then perhaps you should visit the temples at Ostia,” said the Englishman. “Or the Louvre. Or the Frankfurt Archaeological Museum. You might find it educational.”

The American woman turned to him. “You don’t need to be patronizing.”

“Trust me, madam. Nothing our delightful guide here has told us is either shocking or untrue.”

“Now you know as well as I do that Christ was not some Persian guy in a funny hat who kills bulls.”

Lily said, “I only wanted to point out the interesting parallels in the iconography.”

“What?”

“Look, it’s not that important, really,” said Lily, hoping, desperately, that the woman would just let it go, realizing, too, that any hope she had of a generous tip from the American couple had long since vanished. “It’s just mythology.”

“The Bible isn’t mythology.”

“I didn’t mean it that way.”

“What does anyone really know about the Persians, anyway? I mean, where’s their holy book?” The other tourists said nothing, just stood around looking uncomfortable.

Let it go. It’s not worth an argument.

But the woman wasn’t finished yet. Since stepping aboard the tour van that morning, she had complained about everything to do with Italy and Italians. Rome traffic was chaotic, not like in America. The hotels were too expensive, not like in America. The bathrooms were so small, not like in America. And now, this final irritation. She had walked into the Basilica di San Clemente to view one of the earliest Christian meeting places, and instead was getting an earful of pagan propaganda.

“How do we know what the Mithrans really believed?” she asked. “Where are they now?”

“Exterminated,” said the Englishman. “Their temples were destroyed long ago. What do you think happened after the church claimed that Mithras was the spawn of Satan?”

“That sounds like rewritten history to me.”

“Who do you suppose did all the rewriting?”

Lily cut in. “This is where our tour ends. Thank you all very much for your attention. Feel free to linger here if you’d like. The driver will be waiting for you in the van when you’re ready to leave. He’ll take you all back to your hotels. If you have any other questions, I’d be happy to answer them.”

“I think you should let tourists know ahead of time,” the American woman said.

“Ahead of time?”

“This tour was called ‘The Dawn of Christianity.’ But it’s not history. It’s pure mythology.”

“Actually,” sighed Lily, “it is history. But history isn’t always what we’ve been told.”

“And you’re an expert?”

“I have a degree in”-Lily paused. Careful-“I’ve studied history.”

“And that’s it?”

“I’ve also worked in museums around the world,” Lily answered, too annoyed now to be cautious. “In Florence. Paris.”

“And now you’re a tour guide.”

Even in that chilly subterranean room, Lily felt her face go hot. “Yes,” she said, after a long silence. “I’m just a tour guide. Nothing else. Now, if you’ll excuse me, I’ll go check on our driver.” She turned and headed back into the labyrinth of tunnels. She certainly would not be getting any tips today, so they could damn well find their own way back upstairs.

She climbed from the Mithraeum, with each step moving forward in time, ascending to the Byzantine foundations. Here, beneath the current Basilica di San Clemente, were the abandoned hallways of a fourth-century church that had lain hidden for eight centuries, buried beneath the medieval church that later replaced it. She heard voices approaching, speaking French. It was another tour group, on their descent to the Mithraeum. They came through the narrow corridor, and Lily moved aside to let the three tourists and their guide pass. As their voices faded, she paused beneath crumbling frescoes, suddenly feeling guilty that she had abandoned her own group. Why had she let the comments of one ignorant tourist so upset her? What was she thinking?

She turned, and froze as she confronted the silhouette of a man standing at the far end of the corridor.

“I hope she did not upset you too much,” he said. She recognized the voice of the German tourist and released a breath, all her tension instantly gone.

“Oh, it’s all right. I’ve had worse things said to me.”

“You did not deserve it. You were only explaining the history.”

“Some people prefer their own version of history.”

“If they don’t like to be challenged, then they should not come to Rome.”

She smiled, a smile he probably could not see from the far end of the murky tunnel. “Yes, Rome has a way of challenging us all.”

He moved toward her, stepping slowly, as though approaching a skittish deer. “May I offer a suggestion?”