One of the best-known such churches, sometimes called an Ossuary, is the Capuchin crypt beneath the church of Santa Maria della Concezione dei Cappuccini on the Via Veneto in Rome, Italy, which includes six chapels, five of which are covered in the skeletal remains of Capuchin friars of yesteryear.
The crypt was built in the 1630s, when Pope Urban VIII ordered some Capuchin friars to set up residency at the Church, and asked that they bring the remains of their bygone brothers with them, so that they would not be abandoned.
In total, an estimated 4,000 skeletons, from friars deceased between the 1520s - 1870s, decorate the insides of the various chapels. The various crypts include a crypt of the resurrection, a crypt of skulls, a crypt of leg and thigh bones, and a crypt of pelvises. A plaque in on display in the crypt reads: "What you are now, we once were; what we are now, you shall be."
Allegedly, this Roman ossuary inspired a similar "Bone Church" in Prague, in the Czech Republic. There, the Sedlec Ossuary, built by Cistercian monks, is decorated with the remains of an estimated 40,000 people.
The reason for the large number of remains dates back to the 1200s, when a Cistercian monk returned from a pilgrimage to the Holy Land, where he brought back dirt from Golgatha, the hill where Christ was crucified, and sprinkled that dirt in the cemetery at the monastery.
As word of this holy dirt spread, the cemetery became a popular place in which to buried. By the time the plague hit, the number of people requesting burial in the cemetery became so great that the monks began exhuming the bones, storing them in the church, and using them for interior decoration.
The Church has been restored several times and is no longer in possession of the Cistercian order, but the popular site receives thousands of visitors annually.
A third popular "Bone Church" is the Capela dos Ossos, in Évora, Portugal, next to the Church of St. Francis.
Built by a Franciscan in the 16th century, the chapel has similar origins to the Czech Ossuary, in that it became a creative way to store the bones contained in cemeteries running out of room to house remains.
Reportedly, the monk also believed that the Church could be a force for the Counter-Reformation, and a good place for Catholics of the area to come and remember their mortality.
Like the Roman ossuary, the bone church in Portugal has several "memento mori" themed inscriptions, including Ecclesiastes 7:1 "A good name is better than good ointment, and the day of death than the day of birth."
Dominicans - the best order in which to die
For Dominican friars, their "memento mori" comes every day when they recite prayers for the dead, said Fr. Aquinas Guilbeau, professor of moral theology for the Pontifical Faculty of the Immaculate Conception at the Dominican House of Studies.
The Dominicans pray for the dead so frequently that it's become part of a joke, he told CNA.
"There are many reasons you want to live in the other orders - the Benedictines, the Franciscans, the Jesuits - but out of all of them, you want to die with the Dominicans, because we constantly pray for the dead," he said.
Whenever a Dominican friar dies, all the priests in his province celebrate a Mass for him. The order also prays what is called the "De Profundis" - a daily prayer, typically before a main meal, that includes praying Psalm 130 in remembrance of all of the men of the province whose death anniversary is on that day.
Dominicans also celebrate an additional "All Saints Day" and "All Souls Day" - they celebrate these feasts with the Church on Nov. 1 and 2, but then they celebrate a second round of these feasts on Nov. 7 and 8, particularly praying for the Dominican saints and souls.
"In terms of praying daily for the dead, it is a constant reminder of our own mortality, that heaven and eternal life is the goal, and it's also a reminder that death is something that we all face," Guilbeau said.
"When we die, we go alone, there's no one who accompanies us in that at that moment. But by praying for those who have gone before us in death, we get a sense of that union and community that endures into the next life, and insofar as we aid the dead by our prayers, they're waiting for us and aiding us by their prayers. It's a daily reminder of the common prayer that we have for each other."
"In terms of...sleeping in our coffin or having skulls on the desk, we don't do that," Guilbeau said, but he added that the black cape that Dominicans wear is meant to serve as a physical "memento mori" for the order.
The daily reminder of death isn't something "macabre or depressing," Guilbeau added, "but it's something hopeful and joyful, that this veil of tears is not the end of our existence, it's not the goal."
"If we live in the love of Jesus Christ and we live in the light of the Holy Spirit, there's constant preparation and help and grace and strength for that moment when we pass from this life to the next," he said.
Therefore, for the saint, death isn't something to be feared, but welcomed and embraced like a sibling, Guilbeau said, recalling the words of St. Francis who once wrote in his "Canticle of the Sun": "Praised be You, my Lord through Sister Death, from whom no-one living can escape. Woe to those who die in mortal sin! Blessed are they She finds doing Your Will."