The saying "When in Rome, do as the Romans do" was first uttered by St. Ambrose, Bishop of Mediolanum (modern-day Milan). When St. Augustine arrived in Mediolanum in the year 387, he noticed that the Church in Milan did not fast on Saturday as did the Church at Rome. He asked Ambrose about this. Ambrose responded, "When I am at Rome, I fast on a Saturday; when I am at Milan, I do not. Follow the custom of the Church where you are". The comment was changed to "When they are in Rome, they do there as they see done" by Robert Burton in his Anatomy of Melancholy, and still later took its current form. (source)

St. Simeon Stylites (ca. 386–459) spent the last thirty-nine years of his life sitting on top of a pillar 70 feet high. He holds what is now the longest-standing individual record listed in Guinness World Records. (source)

St. Patrick (circa 385–461), who in his youth had been enslaved in Ireland, was the first prominent historical figure to speak out against slavery.

St. Patrick was not Irish. He was British, likely from modern-day Wales, and never set foot in Ireland before he was kidnapped by Irish raiders. After escaping, he became a priest and a bishop and returned to Ireland as a missionary. He was made the patron saint of Ireland due to his success in converting the Irish. (source)

St. Patrick was not the first Christian missionary sent to Ireland. According to Prosper of Aquitaine, Pope Celestine I sent a deacon named Palladius to believers in Ireland in the year 431, a few years before Patrick went to Ireland to begin his mission. (source)

The Navigatio Santi Brendani Abatis, a ninth century manuscript, describes the many adventures of St. Brendan the Navigator, who supposedly undertook a seven-year voyage across the Atlantic Ocean, eventually reaching what might possibly have been Newfoundland. In 1976–77, Tim Severin, a British scholar, crossed the Atlantic on a boat constructed based on the details described by Brendan, demonstrating the feasibility of such a voyage. (source)

The son of Pope Hormisdas (pope from 514 to 523), Silverius, was himself elected pope in 536. Silverius was deposed nine months later and exiled to Palmarola, where he died three months later. Both popes were eventually canonized. (source)

The seven deadly sins (anger, covetousness, envy, gluttony, lust, pride, and sloth) are not found in the Bible; they were first enumerated by St. Thomas Aquinas. (source)

The first female saint formally canonised by the Vatican (as opposed to the older, "pre-congregation" saints that were not formally canonised) was Saint Wilborada, canonised in 1047 by Pope Clement II. She was an anchoress who warned the monks of St. Gall of an impending Hungarian invasion. However, being an anchoress, she was walled into a small cell and could not escape, and so was martyred by the Hungarians. (source)

The legend of the Loch Ness Monster began circa 565, when St. Columba claimed to meet a water beast at Loch Ness and granted it "perpetual freedom of the loch". (source)

St. Edmund the Martyr (841–869), King of East Anglia, was killed at the hands of the Vikings, either by undergoing the blood eagle rite (having his ribs pried open to expose the still-breathing lungs) or by being whipped, shot through with an enormous number of arrows, and being decapitated (source)

The Gregorian chant was named after Pope St. Gregory I.

Many traditions for the holidays of Christmas, Easter, and Halloween/All Saints' Day were created between roughly the 4^th and 7^th centuries to compete with pagan traditions. For example, All Saints' Day was created by fourth-century missionaries as a rival to the Celtic holiday Samhain, with its new traditions designed to portray the rival pagan gods as devils, spirits, and witches.

Slavery ended in Western Europe in the 7^th century, when a British girl, Bathilde, was enslaved and sold to King Clovis II of the Franks (638–655). Clovis fell in love with and married her. After the king died, Bathilde, acting as regent for their three young sons, outlawed slavery. She was later canonized as a saint by the Roman Catholic Church. (source)

St. Cuthbert's death shroud, in Durham Cathedral, reads "There is no God but Allah". In the Middle Ages, much of Europe's silk was imported from Islamic lands; Arabic inscriptions on the silk were often ignored. (source)

Relics of saints were so valued in the Middle Ages that when Elizabeth of Hungary, a holy woman, died in 1231, her body was quickly dismembered for holy relics by a crowd.

In the 11th century, the aged St. Romuald planned to move from his Umbrian town. The residents of the town, fearful that another city would end up with his bodily remains as holy relics, plotted his murder.

St. Thomas Aquinas was once kidnapped by his own family. After studying at Monte Cassino and at the University of Naples, he joined the new Dominican order in 1244. His family objected, kidnapped him, and held him in custody. He escaped, travelling to Paris. His philosophical system remains the basis of Catholic teaching. By upholding reason as a respected method for extending the boundaries of human knowledge, he helped to make science respectable again in Christian Europe. (source)

The writings of St. Thomas Aquinas comprise 25 volumes. Oddly, Aquinas stopped writing in December 1273, three months before he died. While healthy, he had had a mystical experience at prayer one evening, after which he was said to have remarked, "All I have written seems like straw to me." (source)

St. Joan of Arc began hearing voices and having visions when she was 13 years old. When she was 17, she led the French army into battle, and she was burned at the stake before she turned 20 years old.

There were 124 popes (in addition to around 23 "anti-popes") in the second millennium (1001–2000). Only five of these have been canonized as saints. (source)

In the Middle Ages, the skulls of saints were used as drinking cups on ceremonial occasions.

St. Adrian Nicomedia is the patron saint of arms dealers. (source)

Twice yearly, in September and May, thousands of Italians travel to the cathedral of Naples to honour St. Januarius. Only 100 people are allowed inside, where they see several old women shout and mutter at two small phials filled with a brown, crusty substance alleged to be the blood of the saint, who was beheaded by Roman Emperor Diocletian in the year 305. The women, thought to be his "relatives", cheer as his blood changes from a gritty brown colour to a violent, bubbling scarlet. Extensive scientific investigation of this miracle has never been performed, but in 1902 a group of scientists at the University of Naples passed pure white light through the material and found that emerging rays were nearly identical to those produced when light passes through normal blood.

The phrase "devil's advocate" came from the process by which the Roman Catholic Church, prior to 1983, investigated candidates for sainthood. In this process, one person took the role of arguing against canonization by denigrating the potential saint on behalf of the devil. The official title of this role was "Devil's Advocate". (source)

St. Anthony the Abbot (also spelled Antony) is the patron saint of both pig herders and skin diseases. The connection between the two is that pork fat was once used to dress wounds. (source)

The first autobiography is generally considered to be St. Augustine's Confessions.

Only three angels are mentioned in the Bible: Gabriel, Michael, and Raphael (the last of whom is only mentioned in the Book of Tobit, which is accepted as canon only by Catholics and Orthodox). (source)