Presbyterian tradition, particularly that of the Church of Scotland, traces its early roots to the Church founded by Saint Columba, through the 6th century Hiberno-Scottish mission. Tracing their apostolic origin to Saint John, the Culdees practised Christian monasticism, a key feature of Celtic Christianity in the region, with a presbyter exercising "authority within the institution, while the different monastic institutions were independent of one another." The Church in Scotland kept the Christian feast of Easter at a date different from the See of Rome and its monks used a unique style of tonsure. The Synod of Whitby in 664, however, ended these distinctives as it ruled "that Easter would be celebrated according to the Roman date, not the Celtic date." Although Roman influence came to dominate the Church in Scotland, certain Celtic influences remained in the Scottish Church, such as "the singing of metrical psalms, many of them set to old Celtic Christianity Scottish traditional and folk tunes", which later became a "distinctive part of Scottish Presbyterian worship".