A year ago, around the Jewish High Holy Days, Mark Brown was diagnosed with stage 3 colon cancer and thought he wouldn’t make it to 50.

Today, as the holidays reach their climax starting Tuesday evening with Yom Kippur — the holiest day of the year on the Jewish calendar — Brown is alive and doctors say he is free of cancer.

He credits rediscovering his Jewish faith for pulling him through the crisis and helping him recover.

“It was something that kept me together when I first got cancer,” said Brown, 49, a web technologist at Adelphi University who lives in Crown Heights, Brooklyn.

Today “I just have a love of life that is beyond anything I’ve ever experienced before,” he said. “I find myself waking up at four o’clock in the morning and just can’t wait for the sun to come up.”

Yom Kippur, or the Day of Atonement, is the most solemn and important day of the year for Jews. Starting Tuesday at sunset, the faithful abstain from eating, drinking and other comforts for 25 hours.

Brown is among the millions of Jews who will spend the evening and the next day in prayer and intense self-reflection, until the stars come out Wednesday about an hour after sunset.

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“Yom Kippur is a gift of the Jewish imagination that we can transform ourselves and use today to build a better platform for tomorrow for ourselves, our families, our communities and our country,” said Rabbi Howard Buechler of the Dix Hills Jewish Center.

“It’s reflecting upon where we’ve been, to have the courage to say that one is sorry, and to mend one’s ways and to revel in the possibilities of taking steps in the right direction,” he said.

Yom Kippur culminates the High Holy Days, which began this year on the evening of Oct. 2 with Rosh Hashana, the Jewish New Year. The holy day will end with a “triumphal blast” of a shofar, or ram’s horn, Buechler said.

Brown, a native of London who moved to New York City, served in the U.S. Marines and fought in the Gulf War, said he is happy just to be alive. For him, Yom Kippur represents a new beginning.

“It’s another chance, that’s what it is,” he said. “I was dying, headed out of this life.”

Doctors at Long Island Jewish Valley Stream operated on Brown soon after he arrived there last year, after another hospital had misdiagnosed him, he said. Doctors at Valley Stream informed him that a growth in his colon was cancerous.

“A numbness came over me,” Brown later wrote in a letter to the hospital staff thanking them for saving his life. “Cancer is a lonely place, one of the loneliest places I think a person could ever experience.”

Brown said he had fallen away from his Jewish faith years ago, in part because family life got so busy after the birth of his daughter. As his health crisis hit, he rediscovered his faith.

He started visiting the central headquarters of the Chabad-Lubavitch Hasidic movement on Eastern Parkway in Brooklyn for morning prayers, where he was warmly embraced.

“I turned to my religion. It was the only thing I could do,” he said.

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Gradually, amid his chemotherapy treatments — every two weeks for six months — his faith grew stronger and his medical condition improved.

“In the midst of the chemo, I had a breakthrough” and embraced his Jewish faith, he said. “From that moment forward, I’ve just been living life in the moment.”

Dr. Ira Klonsky, the surgeon who operated on Brown, said Brown was in very serious condition when he arrived at Valley Stream in November and needed to go into the operating room immediately. “He was very sick,” Klonsky said.

Today, Brown has completed his treatment and is doing “great,” with the cancer in remission, Klonsky said.

Brown said he plans to spend Yom Kippur at home in Brooklyn praying, since he still does not have the energy he once did. But he is surprised — and grateful — that he is here to mark the holy day at all.

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“Everything is new to me now because I almost died,” he said. “I found the god of Israel in cancer.”