There he wrote in Italian a work entitled "Paschologia" (Cologne, 1702), in which he dealt with the differences of calculation in the calendars of the Greek, Roman, and Jewish churches, and demonstrated the errors which had crept into the calendar from the First Council of Nicaea until 1692.
The First Council of Nicaea in 325 decreed the saints' days, feasts and other holy days, which came to be printed on church calendars in red.
There are just eight churches in Britain named after the Bishop of Poitiers, St. Hilary, who contributed to the First Council of Nicaea in 325 AD, and it is thought that they were founded by St Germanus, who was invited from France as a missionary by the 5th century English church.
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Upon his return, he regained the lost ground resulted from the First Council of Nicaea, established alliances with other groups such as the Meletians and expelled many opponents.
The issue of how to reconcile the claims of monotheism with the assertion of the divinity of Jesus of Nazareth was largely settled at the First Ecumenical Council held at Nicaea (325).
Isaac's great work was the organizing of the Council of Seleucia, the equivalent for the Eastern Syrian Church of the First Council of Nicaea.
The relative penitential canons are contained in the canonical letter of St. Gregory Thaumaturgus (about 263; P. G., X, 1019), the Councils of Ancyra (314), Neocaesarea (314-20), Nicaea (325), and the three canonical letters of St. Basil to Amphilochus (Ep. 188, 199, 217 in P. G., XXXII, 663, 719, 794).
The diplomatic service of the Holy See can be traced back to the First Council of Nicaea when Pope Sylvester I sent legates to represent him during the discussions of the council.