Early Christian Synods & Gatherings Before Nicaea

       The Council of Jerusalem (c. 48–50 AD): Gentile Inclusion and Apostolic Law

This was the first official Council, dating between 48-50. It decreed that Gentiles were, under penalty of law, not required to receive circumcision. Acts 15 relays the outcome, given by James, the Brother of Jesus (also known as James the Just):

"But that we write unto them, that they refrain themselves from the pollutions of idols, and from fornication, and from things strangled, and from blood. For Moses of old time hath in every city them that preach him in the synagogues, where he is read every sabbath. Then it pleased the apostles and ancients, with the whole church, to choose men of their own company, and to send to Antioch, with Paul and Barnabas, namely, Judas, who was surnamed Barsabas, and Silas, chief men among the brethren"             -Acts 15:19-22

The Synod at Rome (155 AD): St. Polycarp and the Calculation of Easter

This meeting discussed the date of Easter; this was the first of many, as the date of Easter was still being discussed some 500 years later at the Synod at Whitby in Great Britain. Indeed, two millennia after the Resurrection, the global Christian community still cannot decide on one specific date each year.  Polycarp of Smyrna attended the 155 Synod in Rome,  It was decided here that Easter would be celebrated on the first Sunday after the first Full Moon after the Vernal Equinox (say that three times fast, lol).

The Synod of Rome (193 AD): Pope Victor I and the Quartodeciman Controversy

Pope Saint Victor I called this meeting to attempt to change when the Eastern Churches celebrated Easter, which was on the 14th day of Nisan each year. Victor's decree, enforced by penalty of excommunication, that all local Churches must celebrate Easter was widely rejected in the East, and Irenaeus of Lyon rebuked Victor for his fanaticism. (much taken from https://4marksofthechurch.com/the-quartodeciman-controversy-the-council-of-rome/). 

The Synod of Carthage (251 AD): St. Cyprian, Novatianism, and the Readmission of the Lapsi

Called under the leadership of Saint Cyprian, this synod addressed the Lapsi. These were Christians who renounced their Faith under the duress of official Roman persecution. Saint Cyprian, and until his death Pope Saint Fabian held these individuals should be welcomed back to the Church after repentance and penance. Their opponent in this matter was the priest Novatian, who believed that only God could forgive those sins and only after death could that occur. The Carthage Synod of 251 showed leniency towards the Lapsi.