First Council of Nicaea

== Separation of Easter computation from Jewish calendar ==
The feast of Easter is linked to the Jewish [[Passover]] and Feast of Unleavened Bread, as Christians believe that the [[Crucifixion of Jesus|crucifixion]] and [[Resurrection of Jesus|resurrection]] of Jesus occurred at the time of those observances. As early as [[Pope Sixtus I]] in the 2nd century, some Christians had set Easter to a Sunday in the lunar month of [[Nisan]]. To determine which lunar month was to be designated as Nisan, Christians relied on the Jewish community. By the late 3rd century some Christians began to express dissatisfaction with what they took to be the disorderly state of the [[Hebrew calendar|Jewish calendar]]. They argued that contemporary Jews were identifying the wrong lunar month as the month of Nisan, choosing a month whose 14th day fell before the [[March equinox|spring equinox]].{{Sfn|Anatolius|loc=Book 7, Chapter 33}}

Christians, these thinkers argued, should abandon the custom of relying on Jewish informants and instead do their own computations to determine which month should be styled Nisan, setting Easter within this independently computed, Christian Nisan, which would always locate the festival after the equinox. They justified this break with tradition by arguing that it was in fact the contemporary Jewish calendar that had broken with tradition by ignoring the equinox and that in former times the 14th of Nisan had never preceded the equinox.{{Sfn|Chronicon Paschale}} Others felt that the customary practice of reliance on the Jewish calendar should continue, even if the Jewish computations were in error from a Christian point of view.{{Sfn|Panarion|loc=Book 3, Chapter 1, Section 10}}

The controversy between those who argued for independent computations and those who argued for continued reliance on the Jewish calendar was formally resolved by the Council, which endorsed the independent procedure that had been in use for some time at Rome and Alexandria. Easter was henceforward to be a Sunday in a lunar month chosen according to Christian criteria—in effect, a Christian Nisan—not in the month of Nisan as defined by Jews.<ref name="ReferenceB" /> Those who argued for continued reliance on the Jewish calendar (called "protopaschites" by later historians) were urged to come around to the majority position. That they did not all immediately do so is revealed by the existence of sermons,{{Sfn|Chrysostom|p=47}} canons,{{Sfn|SEC|p=594}} and tracts{{Sfn|Panarion|loc=Book 3, Chapter 1}} written against the protopaschite practice in the late 4th century.

These two rules—independence of the Jewish calendar and worldwide uniformity—were the only rules for Easter explicitly laid down by the Council. No details for the computation were specified; these were worked out in practice, a process that took centuries and [[Easter controversy|generated numerous controversies]], some of which remain unresolved. In particular, the Council did not seem to decree that Easter must fall on Sunday.{{Sfn|Sozomen|loc=Book 7, Chapter 18}} This was unnecessary as it resolved against the ''Quartodecimani'', who celebrated on any day of the week, in favour of the Churches who postponed the celebration to the following Sunday. See the extract from the ''Letter of the Council of Nicaea to the Egyptian Church'', cited above.

Nor did the Council decree that Easter must never coincide with Nisan 15 (the first Day of Unleavened Bread, now commonly called "Passover") in the Hebrew calendar. The Finnish Orthodox Church explains, "According to the definition of the Council of Nicaea in 325, Pascha is celebrated on the first Sunday after the full moon following the vernal equinox, but always after the Jewish Passover. The date of the vernal equinox was then defined as March 21."<ref>{{cite web|url=http://orthochristian.com/157576.html|last=Finnish Orthodox Church|title=Finnish church resolves to continue using Gregorian Paschalion|date=30 November 2023|accessdate=9 March 2024}}</ref> L'Huillier notes the success of this strategy - Orthodox Easter has never preceded Passover.{{Sfn|L'Huillier|1996|p=25}}