(Hefele: Hist. of the Councils, Vol. I., pp. 328 et seqq.)
The differences in the way of fixing the period of Easter did not indeed disappear after the Council of Nicea. Alexandria and Rome could not agree, either because one of the two Churches neglected to make the calculation for Easter, or because the other considered it inaccurate. It is a fact, proved by the ancient Easter table of the Roman Church, that the cycle of eighty-four years continued to be used at Rome as before. Now this cycle differed in many ways from the Alexandrian, and did not always agree with it about the period for Easter-in fact (a), the Romans used quite another method from the Alexandrians; they calculated from the epact, and began from the feria prima of January. (b.) The Romans were mistaken in placing the full moon a little too soon; whilst the Alexandrians placed it a little too late. (c.) At Rome the equinox was supposed to fall on March 18th; whilst the Alexandrians placed it on March 21st. (d.) Finally, the Romans differed in this from the Greeks also; they did not celebrate Easter the next day when the full moon fell on the Saturday.
Even the year following the Council of Nicea-that is, in 326-as well as in the years 330, 333, 340, 341, 343, the Latins celebrated Easter on a different day from the Alexandrians. In order to put an end to this misunderstanding, the Synod of Sardica in 343, as we learn from the newly discovered festival letters of S. Athanasius, took up again the question of Easter, and brought the two parties (Alexandrians and Romans) to regulate, by means of mutual concessions, a common day for Easter for the next fifty years. This compromise, after a few years, was not observed. The troubles excited by the Arian heresy, and the division which it caused between the East and the West, prevented the decree of Sardica from being put into execution; therefore the Emperor Theodosius the Great, after the re-establishment of peace in the Church, found himself obliged to take fresh steps for obtaining a complete uniformity in the manner of celebrating Easter. In 387, the Romans having kept Easter on March 21st, the Alexandrians did not do so for five weeks later-that is to say, till April 25th-because with the Alexandrians the equinox was not till March 21st. The Emperor Theodosius the Great then asked Theophilus, Bishop of Alexandria for an explanation of the difference. The bishop responded to the Emperor's desire, and drew up a chronological table of the Easter festivals, based upon the principles acknowledged by the Church of Alexandria. Unfortunately, we now possess only the prologue of his work.
Upon an invitation from Rome, S. Ambrose also mentioned the period of this same Easter in 387, in his letter to the bishops of Aemilia, and he sides with the Alexandrian computation. Cyril of Alexandria abridged the paschal table of his uncle Theophilus, and fixed the time for the ninety-five following Easters-that is, from 436 to 531 after Christ. Besides this Cyril showed, in a letter to the Pope, what was defective in the Latin calculation; and this demonstration was taken up again, some time after, by order of the Emperor, by Paschasinus, Bishop of Lilybaeum and Proterius of Alexandria, in a letter written by them to Pope Leo I. In consequence of these communications, Pope Leo often gave the preference to the Alexandrian computation, instead of that of the Church of Rome. At the same time also was generally established, the opinion so little entertained by the ancient authorities of the Church-one might even say, so strongly in contradiction to their teaching-that Christ partook of the passover on the 14th Nisan, that he died on the 15th (not on the 14th, as the ancients considered), that he lay in the grave on the 16th, and rose again on the 17th. In the letter we have just mentioned, Proterius of Alexandria openly admitted all these different points.
Some years afterwards, in 457, Victor of Aquitane, by order of the Roman Archdeacon Hilary, endeavoured to make the Roman and the Alexandrian calculations agree together. It has been conjectured that subsequently Hilary, when Pope, brought Victor's calculation into use, in 456-that is, at the time when the cycle of eighty-four years came to an end. In the latter cycle the new moons were marked more accurately, and the chief differences existing between the Latin and Greek calculations disappeared; so that the Easter of the Latins generally coincided with that of Alexandria, or was only a very little removed from it. In cases when the id' fell on a Saturday, Victor did not wish to decide whether Easter should be celebrated the next day, as the Alexandrians did, or should be postponed for a week. He indicates both dates in his table, and leaves the Pope to decide what was to be done in each separate case. Even after Victor's calculations, there still remained great differences in the manner of fixing the celebration of Easter; and it was Dionysius the Less who first completely overcame them, by giving to the Latins a paschal table having as its basis the cycle of nineteen years. This cycle perfectly corresponded to that of Alexandria, and thus established that harmony which had been so long sought in vain. He showed the advantages of his calculation so strongly, that it was admitted by Rome and by the whole of Italy; whilst almost the whole of Gaul remained faithful to Victor's canon, and Great Britain still held the cycle of eighty-four years, a little improved by Sulpicius Severus. When the Heptarchy was evangelized by the Roman missionaries, the new converts accepted the calculation of Dionysius, whilst the ancient Churches of Wales held fast their old tradition. From this arose the well-known British dissensions about the celebration of Easter, which were transplanted by Columban into Gaul. In 729, the majority of the ancient British Churches accepted the cycle of nineteen years. It had before been introduced into Spain, immediately after the conversion of Reccared. Finally, under Charles the Great, the cycle of nineteen years triumphed over all opposition; and thus the whole of Christendom was united, for the Quartodecimans had gradually disappeared.(2)