In the year 563 Columban came with twelve men from Ireland to Iona and founded there the monastery Iona Abbey. From Iona Christianity spread in the form of the so-called Celtic Church in Scotland and northern England. Presumably, the Book of Kells, one of the most valuable works of insular illumination, was written in the 7th or 8th century on Iona. Iona was the spiritual center of the Celtic Church, to whose followers the Culdeer counted.
Iona was considered a "holy place" for several centuries. Scottish Irish and Norwegian kings are to be buried in the cemetery Reilig Odhráin. So Iona was burial place of the kings of Dalriada and their successors, the early kings of Scotland. The last of these was Macbeth, known from Shakespeare's drama of the same name. Numerous crosses were created at this time on Iona. Many were later thrown into the sea by puritanical groups. The St. John's Cross from the 8th and the St. Martin's Cross from the 9th century still exist, with some fragments.
The Celtic monastery existed until the 11th century. It was destroyed by Viking raids (795, 802, 806). In the 12th century, the Culdeer were subject to the northward-pressing followers of the Roman Catholic Church. As a result, a second monastery was built in 1203 as a Benedictine abbey and cathedral of the islands. The monastery existed until the Reformation, then fell and was rebuilt in 1939. Source: Wikipedia