Have you never heard of the Festal Letters of St. Athanasius?
You must have missed my reply #2288 above in which I mentioned them.
St. Athanasius did fulfill the mandate of the council and sent Festal Letters to inform the other Churches when to celebrate Pascha.
No, he didn't. The festal letters were written to the bishops of Egypt, Libya, and Cyrenaica, and nowhere else. In the words of the Aramaic index to the letters, "he sent [them] year by year to the several cities and all the provinces subject to him
; that is, from Pentapolis, and on to Libya, Ammoniaca, the greater and the lesser Oasis, Egypt, and Augustamnica, with the Heptanomis and the upper and middle Thebais." Map:http://upload.wikimedia.org/wikipedia/commons/e/e2/Dioecesis_Aegypti_400_AD.png
The conflict between Rome and the Churches in Asia Minor was over when to celebrate Pascha led to the decision of the 1st Ecumenical Council, Nicaea I in 325. Rome celebrated Pascha on the Sunday after the Jewish Passover which fell on the day of the first full moon following the Spring Equinox. The Churches in Asia Minor celebrated Pascha on the day of the Jewish Passover regardless of the day of the Week.
So you keep saying, but the source documents say otherwise. The controversy at Nicea had nothing to do with the churches of the Province of Asia. Athanasius is quite clear on who the hold-outs for the old custom were: "They of Syria, Cilicia, and Mesopotamia
differed from us, and kept the feast at the same season as the Jews" (Ad Afros
2). The Province of Asia is not mentioned. And the controversy is about the "season", not the weekday. Apostolic Canon 7, which you quote, corroborates Athanasius's statement that the controversy at Nicea was about the "season": "If any bishop, presbyter, or deacon, shall celebrate the holy day of Easter before the vernal equinox, with the Jews,
let him be deposed." At that time there was no unified Jewish calendar. Jews in at least some places would sometimes identify as Nisan a lunar month whose full moon fell before the equinox. Christian critics said they were not "observing the month of the new" (Deut. 16.1) because they were holding their Matzoth in the last month of the old year. Those Christians who celebrated "at the same season as the Jews" would therefore necessarily sometimes have Easter before the equinox as well.
The consensus reached by the Eastern Orthodox after the Aleppo proposal of 1997 by a committee of the World Council of Churches that all Christians celebrate Pascha on the Sunday following the first full moon following the actual Spring Equinox was that the proposal was unacceptable because failed to include a provision following this canon which according to the consensus of Orthodox scholars requires that Pascha be celebrated after the Jewish Passover. The Gregorian calculation of Pascha allows the Western Easter to take place during or before the Jewish Passover.
But earlier, you wrote
According to Nicaea I, Pascha must follow the Jewish Passover, which was celebrated on the 14 of Nissan, which was the day of the first full moon following the Spring Equinox.
If the "Jewish Passover" that Easter must "follow" is "the day of the first full moon following the Spring Equinox", then the Gregorian Easter always follows the "Jewish Passover" defined in this way. Yet elsewhere you contradict yourself and state that the Gregorian Easter can come before what you call the "Jewish Passover" Why are you inconsistent in your definition of "Jewish Passover"?
Rome did not accept the Alexandrian calculations until 457.
In the mid-5th century, Rome began using the Victorian computus,
which was a 19-year cycle that sometimes differed from the Alexandrian. Only in the 6th century, after receiving the report written by Dionysius Exiguus, did Rome adopt the Alexandrian computus.
The difference was that Alexandria used a 19 year cycle, while Rome used an 84 year cycle. The British clung to the older Roman 84 year cycle until the Synod of Whitby in 664.
The churches of Britain and Ireland used the Celtic-84, a fifth-century variant of the Roman-84, not the original Roman-84 itself. The Synod of Whitby applied only to the northern English, who had been taught the Celtic-84 by their Irish teachers--not to anyone else in Britain. The southern English were already using the Dionysian/Alexandrian computus. The inhabitants of Britain who were not subject to the English, or some of them, as well as the northern Irish, continued using the Celtic-84 for some time thereafter.
The Eastern Orthodox Church continues to obey the First Ecumenical Council and uses the Alexandrian tables for the date of Pascha.
The Alexandrian tables no longer work as they were intended to. Slavish adherence to them is nothing but hollow legalism. If a hand-calculator gave a value of 37.6713 for the sine of some angle, would you not deem it broken and replace it? Your Alexandrian tables are a broken calculator. It is the Gregorian calendar that sets Easter where the fathers of the 3rd and 4th centuries intended it to be.