For almost as long as there have been Hebrew teachers, there’s been this story: A friend of a friend (of a friend) was as Catholic as the pope, but she would light candles in the basement every Friday night and couldn’t explain why.

Every time the story ends the same way: Turns out that friend of a friend was the great-times-100 granddaughter of Conversos, Spanish Jews forced to convert to Catholicism during the Spanish Inquisition.

Many of the Conversos still practiced their religion in secret — hence the basement candlelighting. Over time, most families forgot they had ever been Jewish, but the traditions lived on. No one remembered exactly why they did them. But to the trained eye, the customs were unmistakably the mark of a Jewish lineage.

That’s the basic premise of Jewish author Kathryn Lasky’s young adult novel “Blood Secret.” And it doesn’t take long for the skeletons to start making their appearances.

“Blood Secret” starts off with 14-year-old Jerry Luna, described by doctors as an “elective mute.” That is, she can talk, she just hasn’t — not for six years, ever since her hippie mother vanished somewhere among the red rocks of the American Southwest.

Fed up with her silence, the orphanage Jerry calls home eventually ships her off to her 94-year-old great-aunt Constanza, a lively Catholic baker in Albuquerque, N.M.

Constanza has some strange customs — like never drinking milk with meat (“it’s terrible for your digestion”) and always lighting two candles on Friday night (“to remember the death of Christ”). To Jerry, these are just bizarre old superstitions — but if you paid attention in Hebrew school, you can probably guess there’s something more.

One night, Jerry discovers the trunk in Constanza’s basement. Inside lies the story of her past — starting with Miriam Sanchez in 14th-century Spain, who introduces Jerry to a side of her family that had been lost for centuries.

From there, Lasky deftly moves the narrative between Jerry’s emotional awakening in modern-day New Mexico and her ancestors’ compelling stories of their troubled lives as Conversos in Spain, Mexico and America.

“Blood Secret” is less than 300 pages, yet it feels epic — and mesmerizing. With her haunting, evocative writing, Lasky tells the story of the Inquisition — and what happened after — in a way that is almost like poetry.

And she tells it like it was. This is no “Candide” — Lasky’s auto-da-fé, in which 12-year-old Spaniard Luis Perez sees his parents killed in 1481, is described in horrific, graphic detail. There are narrow escapes, torture, torment and death. The book’s HarperTeen imprint isn’t just a brand name — it’s a warning label. “Blood Secret” is definitely not for children.

Some elements of the story require a swift suspension of disbelief, like Jerry’s discovery of a swatch of lace that’s at least 600 years old yet, remarkably, is still perfectly intact. A coincidence involving a mezuzah is particularly unbelievable — and makes the connection between Jerry and her ancestors feel, momentarily, contrived.

But Lasky’s almost hypnotic prose will keep readers enthralled until the end. The historical characters — particularly Zayana, an Aztec woman who finds happiness in her dead husband’s Jewish rituals — are so compassionately written that their struggles are almost unbearably upsetting. Compared to her ancestors’ tribulations, Jerry’s mommy issues seem somewhat trivial — but that may be the point.

Lasky’s books are often critical of religion — the Amish in “Beyond the Divide,” evangelical Christians in “Memoirs of a Bookbat” — and it’s a special treat to find her writing so lovingly about Judaism. “Blood Secret” is a beautiful paean to a faith that has survived it all.

“Blood Secret” by Kathryn Lasky (256 pages, HarperTeen, $16.89).

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