The earliest Christians, being converted Jews, followed the Jewish practice of observing the day on which the Paschal lamb was slaughtered at Passover, which was the 14th day of the Jewish lunar First Month, Nisan. By the second century Christians of Asia Minor kept Nisan 14, irrespective of what day of the week it fell on, thereby acquiring the name of "Quartodecimans," later to become a term of anathema. The paradox of a Jewish-Christian practice, traced by its adherents back to the Apostle John and acknowledged even by its opponents to be the older practice in the Church, subsequently being declared a heresy was not lost on the later Irish and English churches, in which controversialists (including the 7th-c. Roman curia) professed to detect residual traces of the practice.
By the time Christianity reached Ireland in 431-432, Christians everywhere had agreed that Easter should be kept on a Sunday, following the 14th day of the First Month (luna XIV), after the spring equinox. These early Christians, following the practices of Greek-speaking gentiles, also supposed that pascha derived frompaschein "to suffer," and therefore concluded that Easter denoted the Passion, rather than the Resurrection. The combination of these uncertainties with the fact that churches in Rome and Alexandria (and elsewhere too) differed in their methods of calculating luna XIV and the First Month, as well as the correct date of the equinox, led to the situation in which churches in different parts of the Christian world celebrated Easter Sunday on different dates.
Stark divergences in 444 and again in 455 between the Roman and Alexandrian cycles led Pope Leo to ask his archdeacon, Hilarus, to commission a new table, which was drawn up by the Aquitanian mathematician Victorius and published in 457. Victorius’s incompetence made an awkward situation impossible,and the result was chaos. Two advantages of his table, however, enabled it to secure widespread adherence: it was a perpetual cycle of 532 years (running from c.e. 28 to c.e. 559), and it followed familiar Roman practice by starting the year on January 1. Although Pope John I commissioned another study in 529 with a view to solving the persisting problems, Victorius’s faulty tables were declared the official tables for the Gallican church in 541 (significantly enough, 84 years after their first appearance). While Pope John’s attempted reform resulted in the publication in 525 of the 19-year tables of Dionysius Exiguus (which ran for 95 years from 532-626) based on (the correct) Alexandrian principles, Victorius’s tables continued to be used for several centuries afterward.
It is not certain which Easter cycles were introduced into the fifth-century Irish church, but it may be assumed that Palladius introduced whatever cycle was prevalent in Gaul in the 430s (either an 84-year cycle or an early version of the Alexandrian 19-year "Metonic" cycle championed in the 390s by Ambrose of Milan), while St. Patrick (supposedly active in Ireland at around the same time as Palladius, or perhaps a generation or so later), would, in all likelihood, have introduced a form of the 84-year Easter cycle then in use by the British church. Victorius’s tables were certainly known in Ireland by the sixth century, and when Columbanus of Bangor traveled from Ireland to Gaul circa 590 his realization that they were the standard tables there, apparently sanctioned by Rome (because archdeacon Hilarus had since succeeded Leo as Pope), occasioned his famous first letter addressed to Pope Gregory I—"a letter equally remarkable for baroque Latinity and studied insolence"—in which he damned the Aquitainian’s tables by declaring that they had been dismissed by Irish computists and scholars as being "more worthy of ridicule and pity than of authority." Columbanus told Gregory that his fellow countrymen used an 84-year Easter table and a related tract De ratione paschali attributed to Anatolius, Bishop of Laodicea, in modern Syria (f c. 282). The table was apparently used by all churches in Ireland by that time, and it remained in use by the community of Iona (off the western coast of Scotland) until 716, after which it disappeared without trace. Anatolius, for his part, was cited by controversialists throughout the sixth and seventh centuries (though some suspected the text was a forgery). The rediscovery and publication of the long-lost "Irish 84" in 1985 has for the first time allowed a correct reconstruction of the historical Irish Easter dates, and thereby cleared up many misconceptions about the controversy.
Papal suspicions of Irish Easter practices came to a head in 628-629, when Pope Honorius I addressed a letter to the Irish clergy, admonishing them for theirerroneous ways and urging conformity with Gallican and Roman usages. A synod of southern Irish clerics, which was convened in response to the papal letter, is described in the famous Paschal Letter of Cummian (632-633), without doubt the most important Irish document surviving from this period. Cummian and his colleagues appear to have adopted the Victorian tables (though not before a delegation sent to Rome reported back on their findings), which provoked a letter of response from Segene, abbot of Iona, accusing them of heresy. The fierceness of the language in Cummian’s Letter indicates how heated the debate had become in Irish circles. Not all Irish churches followed Cummian’s party, however. In 640 a group of northern Irish churches (headed by Armagh) wrote to Rome seeking papal advice on how to reckon the Easter date for 641. As it happened, both Victorius and the Irish 84-year Easter table gave Easter Sunday on April 1 of that year, whereas the Dionysiac table gave April 1 as luna XIV, a date on which Easter Sunday was not allowed to fall in the Alexandrian reckoning (which had Easter Sunday therefore on April 8 that year). The same problem arose also in Visigothic Spain, and a letter of Bishop Braulio of Saragossa in response to an unknown enquirer may possibly have been addressed to an Irish correspondent.
It is not known whether Armagh and the northern churches changed their observances after 641, but such evidence as exists suggests that they did not. Certainly, the community of Iona, which from its foundation in 563 was the dominant church in Scotland, and which from 634-635 was in control also of all the newly established churches in the north of England, and whose paruchia included important houses in both Ireland and Britain, held fast to the 84-year Easter tables of its founder, Colm Cille. This in turn led to difficulties, first in the north of England and subsequently in Scotland, which came to a head at the famous synod of Whitby (Northumbria) in 664. The exact nature of the conflict is unclear, but our principal source of information, the Venerable Bede (Ecclesiastical History of the English Nation, III 25) presents the debate as one between Irish traditionalists, partisans of the old 84-year tables, and Romanist "reformers" led by Wilfrid of Hexham and York, who advocated adoption of the Dionysiac tables. The decision of the presiding king, Osuiu, was against the Irish, whose leader, Bishop Colman of Lindisfarne, decided to withdraw from Northumbria and retire back to Iona (and eventually Ireland) along with those of his community (both Irish and Anglo-Saxon) who wished to remain loyal to the old ways.
Only Iona itself amongst Irish foundations appears to have held out in the struggle. Even here, however, after many years’ effort by the Englishman Ecgberct, the island community finally relented and in 716 Easter was celebrated there in accordance with the new (Dionysiac/Alexandrian) ways. After the expulsion of Iona monks from the Pictish kingdom in 717, the only insular churches to resist change after that were the British, some of whom, perhaps in the kingdom of Strathclyde, had come into line already in 703-704 (Bede, Ecclesiastical History of the English Nation V 15) while the last of them, led by Bishop Elfoddw of Bangor, conformed in 768.
