The First Council of Constantinople ("Constantinople I") was called by Emperor Theodosius in A.D. 381 to combat the Arian heresy, which had not ended after being condemned at the Council of Nicaea; and also to combat the heresy of the Macedonians, who denied the divinity of the Holy Spirit.
- "The profession of faith of the holy fathers who gathered in Nicaea... is to remain in force. Every heresy is to be anathematised..."
- "Diocesan bishops are not to intrude in churches beyond their own boundaries..."
- "Because it is new Rome, the bishop of Constantinople is to enjoy the privileges of honour after the bishop of Rome." This canon provoked much controversy later when the Patriarchs of Constantinople used it to try to diminish the authority of the Pope.
- "Maximus the Cynic... never became, nor is he, a bishop... Everything that was done both to him and by him is to be held invalid."
- "Regarding the Tome of the Westerns..." (Unfortunately, this tome has been lost.)
- Only orthodox Christians are allowed to bring accusations against bishops, and then only before the other bishops of the province or diocese.
- Certain heretics are to be received immediately upon confession of their errors; others must be rebaptized after following through the regular process.
At the conclusion of the council, Emperor Theodosius declared that all bishops must confess the equal divinity of all three Persons of the Trinity and be in communion with certain of the bishops who attended. The Creed of Nicaea-Constantinople in later centuries became the standard of Christian orthodoxy and Trinitarian profession. Saint Patrick the Apostle of Ireland famously used the shamrock in the fifth century to explain the doctrine of the Trinity declared at the Council of Nicea and this First Council of Constantinople in the fourth century.