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 distinctions 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". Development Presbyterian history is part of the history of Christianity, but the beginning of Presbyterianism as a distinct movement occurred during the 16th-century Protestant Reformation. As the Catholic Church resisted the Reformers, several different theological movements splintered from the Church and bore different denominations. Presbyterianism was especially influenced by the French theologian John Calvin, who is credited with the development of Reformed theology, and the work of John Knox, a Scottish Catholic Priest who studied with Calvin in Geneva. He brought back Reformed teachings to Scotland. The Presbyterian church traces its ancestry back primarily to Scotland. In August 1560, the Parliament of Scotland adopted the Scots Confession as the creed of the Scottish Kingdom. In December 1560, the First Book of Discipline was published, outlining important doctrinal issues but also establishing regulations for church government, including the creation of ten ecclesiastical districts with appointed superintendents which later became known as presbyteries. In time, the Scots Confession would be supplanted by the Westminster Confession of Faith, and the larger and shorter catechisms, which were formulated by the Westminster Assembly between 1643 and 1649. Characteristics Presbyterians distinguish themselves from other denominations by doctrine, institutional organisation (or "church order") and worship; often using a "Book of Order" to regulate common practice and order. The origins of the Presbyterian churches are in Calvinism. Many branches of Presbyterianism are remnants of previous splits from larger groups. Some of the splits have been due to doctrinal controversy, while some have been caused by disagreement concerning the degree to which those ordained to church office should be required to agree with the Westminster Confession of Faith,