Very little documented information is known about the life of St. Andrew. From the Bible, comes the story of Andrew, a fisherman from Galiliee, brother to Simon, whom Jesus would call peter, and one of the first to be called as a disciple of Christ. Andrews was believed to have been a missionary to Asia Minor and Greece, and was reportedly crucified by the Romans on an x-shaped cross at Patras, in 69 AD, as he did not feel worthy to be crucified on the cross like Christ was. His remains were entombed and in 370 AD, taken from Constantinople (where the bones had resided under the order of the Emperor Constantine) to a Pictish settlement on the eastern coast of Scotland by Saint Rule, who was told in a vision to take the bones to the ‘ends of the earth’ for safe-keeping, and he removed a tooth, arm bone, kneecap and some fingers from the tomb in Constantinope. The settlement later became known as St. Andrews, and the relics were placed first in a small chapel, and then later in the Cathedral of St. Andrews, a center for medieval religious pilgrims...It is believed the relics were destroyed during the Scottish Reformation. The larger part of St. Andrew’s remains were stolen from Constantinople in 1210 and were moved to Amalfi, in southern Italy. In 1879 the local Archbishop sent part of the saint’s shoulder blade to the Scottish Roman Catholic community, and Pope Paul VI presented further relics of the Saint in 1969, which are currently on display in St. Mary’s Roman Catholic Cathedral in Edinburgh.

In 832 AD, a Pictish army under King Angus MacFergus, High King of Alba, along with a force of Scots under Eochaidh, King of Dalriada (and grandfather of Kenneth MacAlpin, was battling a Northumbrian force in Lothian...The night before battle, Saint Andrew reportedly appeared to Angus in a vision, and on the field of battle the next day, a saltire, or x-shaped cross, similar to the one that Saint Andrew was crucified on, appeared in the sky, encouraging the Picts and Scots in their fight and causing the Northumbrians to flee the field...The colors of the flag are supposed to represent the white of clouds and the azure color of the sky. From that time onward, the Saltire became the national emblem of the Scots, not only as a flag, but also worn on tunics and bonnets of Scottish soldiers as a way to identify themselves on the battlefield.
Even during the days of the Scottish Reformation, when Presbyterian reformers sought to remove vestiges of the Catholic Church in Scotland, only the Saltire remained.
Information taken from an article on St. Andrews and His Flag, written by Todd Wilkinson. The full article is available on the Internet scotshistoryonline.co.uk/saltire/saltire.html.
St. Andrew depiction by El Greco.