As Siongco remembers it, her little brother showed signs of a vocation even before starting elementary school. He was fascinated by religious processions and was already singing in church by age 3.
"He loved the saints, he loved praying, he loved singing, he loved everything about the Church," said Siongco.
By the time he had finished high school in 1994, he had broken up with his girlfriend at the time with the intention of entering the seminary.
Those closest to Father San Juan say his life was marked above all else by a life-and-death experience during that time: a testicular cancer diagnosis in 2002 a few months before his ordination to the diaconate.
Chemotherapy left him hairless, pale, and thin, but he vowed to follow through with his ordination to the diaconate. Family, friends, fellow seminarians, and even professors rallied behind him in prayer, and the cancer went into remission in 2003. He was ordained to the priesthood the following year.
"This is my second life, no doubt," Father San Juan told Manila's Phillipine Sunday Inquirer Magazine in an interview after his ordination in 2004. "I see myself in the hands of a loving Father. A second life is his revelation to me that I have a mission to do in His Name."
In the same interview, the new priest shared that the cancer battle had given him more joy and a stronger faith.
"Life will not always be a journey of certainty, of controlling it the way we plan it," he continued. "Doubts and so-called trials will come. But if we seek God in all things, then we learn that God's love is everywhere."
The priest credited his "second life" in particular to Divine Mercy, the Virgin Mary, and the miraculous intercession of St. Thérèse of Lisieux, to whom he had a fervent devotion for the rest of his life.
After spending six years ministering in parishes and schools in Manila, Father San Juan transferred to the Archdiocese of Los Angeles in 2010 to be closer to his family. He served in several parishes, and was officially incardinated as a priest of the archdiocese in 2015.
Among his brother priests, Father San Juan was known as a "holy priest who had a wonderful sense of humor, and always had a smile on his face," according to Vicar for Clergy for the Archdiocese of Los Angeles Msgr. Jim Halley.
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Auxiliary Bishop Alex Aclan remembered how shortly after arriving in the archdiocese, then-Msgr. Aclan relied on Father San Juan twice to write the music for two fundraiser musical plays benefitting the Filipino Priests Association of Los Angeles.
And during the annual Christmastime Simbang Gabi Mass at the Cathedral of Our Lady of the Angels, it was Father San Juan who was charged with leading his brother Filipino priests in singing in Tagalog after Communion.
"That's how he endeared himself to the Filipino priests here," recounted Bishop Aclan. "He was an excellent composer, pianist, and vocalist."
One of those priests, Father Rizalino "Riz" Carranza, spent four years with him at St. Peter Claver Church in Simi Valley, where Father Carranza is pastor and Father San Juan served as associate pastor from 2015 to 2019. He said Father San Juan was the ultimate "people priest," a gifted preacher whose enthusiasm while celebrating the Eucharist was infectious.
"He really appealed to a lot of people of different ages, from the older to the younger," recalled Father Carranza.
In private, his former pastor says Father San Juan was a man of deep prayer. Walking past the door to his room, Father Carranza would sometimes catch a glimpse of Father San Juan on his knees with a lit candle burning.