Though we can't know for certain, St. Augustine is often credited as the source of the quotation, "He who sings, prays twice."

The Benedictines of Mary, Queen of the Apostles, a monastic order of nuns based at the Priory of Our Lady of Ephesus in Gower, Missouri, surely carry that saying to heart when they gather each day in choir.

Their humble efforts, which have resulted in five albums of celestial chants and hymns, are not only reaching the heavens -- they're reaching the Billboard charts. A new PBS documentary called "Lent at Ephesus," to air in June, captures their unlikely road to popular success.

The lives of the nuns are almost disarming in their holy simplicity. Devoted to the pursuit of union with God and guided by the Rule of St. Benedict, they spend their days in prayer, in quiet, in work and in solitude. But they also gather in song -- sometimes up to five hours a day.

Three of their albums -- 2012's "Advent at Ephesus," 2013's "Angels and Saints at Ephesus" and 2014's "Lent at Ephesus" -- have been met with critical and commercial acclaim. Two of their recordings reached number one on the Classical Traditional Billboard chart. Billboard even crowned the sisters the Top Artists in their category two years in a row.

Not bad for a group of people who spend a good portion of their days in silence.

In a subdued, measured way, the documentary, "Lent at Ephesus," captures the nuns' austere lives on their picturesque Midwestern farmland. Their prioress and music arranger, Mother Cecilia, factors most prominently in the film, and she shares a bit of her own remarkable back story.

She was educated at Rice University's Shepherd School of Music in Houston, and at one time was a member of the horn section of Ohio's Columbus Symphony Orchestra. Mother Cecilia, though, felt called to something greater and so entered religious life.

Her perspective frames the documentary and gives the audience a window into a normal day at the community, where unshakable devotion to prayer and the love of worship through singing prevail.

A feature-length treatment would have yielded a deeper examination into their calling to such a simple life -- as well as the impetus for their musical enterprises. But at a thin 45 minutes, audiences are left inspired, humbled, and likely wanting more.

These women have clearly filled a need among music lovers for uplifting, holy melody. What compels them to sing? How does this experience draw them closer to God? Do they know how far their voices carry?

One of the strongest elements of "Lent at Ephesus" is the heavenly sound of the choir that the filmmakers have freely peppered throughout. Sometimes the narration is halted entirely to allow the nuns' voices alone to hold our attention.

Catholic audiences young and old are in for a treat with this modest glimpse into the lives of these holy women and the music they create in praise of God. Together, they're a real-life sister act to treasure.

"Lent at Ephesus" is faith-affirming, quality viewing for all ages. It airs throughout June on PBS stations nationwide; check local listings.